Image forming method and image forming apparatus

ABSTRACT

An image forming method exposes a surface of an image bearer with light according to an image pattern including an image portion to form an electrostatic latent image, and includes: setting, among pixels constituting the image portion, at least a group of pixels existing at a boundary with respect to a non-image portion as a non-exposure pixel group, and at least a group of pixels existing at a boundary with respect to the non-exposure pixel group as a high power exposure pixel group; specifying, among the pixels constituting the image portion, a predetermined pixel as a target pixel, and a group of pixels existing at a boundary with respect to the non-image portion close to the target pixel as a boundary pixel group; and specifying a light power value of the target pixel based on image identification information acquired from the pixels constituting the image portion.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2015-097140, filed on May 12, 2015, the contentsof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image forming methods and image formingapparatuses.

2. Description of the Related Art

In recent years, in an electrophotography process for forming images,demands for high image quality and high stabilization have beenincreased.

Herein, in the electrophotography process, as a method of realizing highimage quality, there is a method of reducing a beam size for exposure.According to the method of reducing the beam size for exposure, smallelectrostatic latent images are formed, so that resolution can beincreased.

However, controlling an image height of the formed electrostatic latentimage while reducing the beam size for exposure is difficult and causeshigh cost in image formation.

In addition, the cost of the controlling the image height of the formedelectrostatic latent image while reducing the beam size for exposureaccounts for a large fraction of the entire cost of the image formingapparatus.

Therefore, in the electrophotography process, there has been arequirement for forming micro-sized electrostatic latent images withoutreducing the beam size for exposure.

In addition, in the image forming method of the related art, a lineimage and a solid image are different in toner adhesion amount height,that is, a pile height. The difference in pile height is caused from adifference in size itself of the electrostatic latent image.

As described above, in consideration of the demand for improvement ofimage quality and the demand for reduction of environmental burden suchas reduction of toner usage or reduction of power usage, there has beena need for appropriately controlling the pile height.

Here, in the case of controlling the pile heights of the line image andthe solid image, a method of performing treatment in a developingprocess is conceivable.

However, since the line image and the solid image have a difference insize itself of the electrostatic latent image, in order to control thepile height in the developing process, sensitivity of the latent imageof the line image and sensitivity of the latent image of the solid imageneed to be differentiated to perform developing.

That is, the method of setting the sensitivity of the latent image ofthe line image and the sensitivity of the latent image of the solidimage to be different from each other to control the pile height is notpreferred because there is a problem such as a loss in fidelity of thelatent image.

As described above, in the image formation, controlling the pile heightwithout performing treatment in the developing process is desired. Inaddition, in the image forming method, desired is a method of forming anelectrostatic latent image such that the difference between image dataand output image occurring in the electrophotography process, other thanthe pile height, can be offset.

In addition, there is disclosed a technique where, in the imageformation, in the case where an area of input image is smaller than apredetermined value, exposure energy per unit pixel is set to be higherthan the exposure energy per unit pixel at the time of solid imagewriting (for example, refer to Japanese Patent Application Laid-open No.2005-193540).

In addition, there is disclosed a technique where, in the imageformation, exposure pixels is thinned out or the exposure pixels isadded to correct light energy exposed from respective light sources tobe uniform (for example, refer to Japanese Patent Application Laid-openNo. 2007-190787).

In the image formation, in the case where a dot density is high, forexample, 1200 dpi, there is a demand for an output image wheremicro-sized characters corresponding to two or three points,particularly, an outlined character of a reversed image of two or threepoints can be recognized.

However, in the image formation, although developing, transfer, andfixing processes are improved in order to output a high-quality imagewith a high dot density, the outputting the high-quality image has beendifficult.

Here, although the measuring the electrostatic latent image in amicrometer scale was difficult, in recent years, the measurement of theelectrostatic latent image at a high accuracy has been enabled. As aresult, it is found out that degradation factor in the image formationis generated in the latent image stage before the developing.

That is, it is found out that, even though the reversed image is outputusing the image pattern as is, a latent image electric vector in avertical direction of a sample occurring in a normal image is notreversed, and a vector of the reversed image is smaller than a vector ofthe image pattern.

Therefore, in the image formation, in the case of outputting amicro-sized image with a high dot density, discrepancy occurs betweenthe latent image and an image pattern signal supplied from a controllerdue to influence of the beam size or charge diffusion. For this reason,in the image forming method, in the case of outputting a micro-sizedimage with a high dot density, even if the developing, transfer, andfixing processes are improved, a high-quality image could not be output.

Particularly, in order to print a character of the reversed image with ahigh image quality, it is effective to increase the latent imageelectric vector in the vertical direction of the sample towards such aside that toner is made not to adhere. In terms of electromagnetism, thesimplest method of increasing the electric vector in a white portion isto increase a charge amount in a white image portion. However, it isdifficult to locally increase an electrification charge amount.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an aspect of the invention, an image forming method exposesa surface of an image bearer with light according to an image patternincluding an image portion and a non-image portion to form anelectrostatic latent image corresponding to the image pattern. The imageportion includes a plurality of pixels. The method includes setting,among the pixels constituting the image portion, at least a group ofpixels existing at a boundary with respect to the non-image portion as anon-exposure pixel group. The method further includes setting, among thepixels constituting the image portion, at least a group of pixelsexisting at a boundary with respect to the non-exposure pixel group as ahigh power exposure pixel group where exposure is performed with lightof a higher light power value than a predetermined light power valuerequired for exposing the image portion. The method still furtherincludes specifying, among the pixels constituting the image portion, apredetermined pixel as a target pixel. The method still further includesspecifying, among the pixels constituting the image portion, a group ofpixels existing at a boundary with respect to the non-image portionclose to the target pixel as a boundary pixel group. The method stillfurther includes acquiring image identification information identifyinga type of the image pattern from the pixels constituting the imageportion. The method still further includes specifying a light powervalue of the target pixel based on the image identification information.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a central cross-sectional view illustrating an embodiment ofan image forming apparatus according to the present invention;

FIG. 2 is a schematic diagram illustrating a corotron type chargingdevice of the image forming apparatus;

FIG. 3 is a schematic diagram illustrating a scorotron type chargingdevice of the image forming apparatus;

FIG. 4 is a schematic view illustrating an example of an opticalscanning device constituting the image forming apparatus;

FIG. 5 is a schematic view illustrating an example of a light source ofthe optical scanning device;

FIG. 6 is a schematic view illustrating another example of the lightsource of the optical scanning device;

FIG. 7 is a block diagram illustrating an image processing section ofthe image forming apparatus;

FIG. 8 is a block diagram illustrating an image processing unit of theimage processing section;

FIG. 9 is a circuit diagram illustrating a light source driving unitconstituting the image forming apparatus of FIG. 1;

FIG. 10 is a block diagram illustrating a light source driving controlunit of the light source driving unit of FIG. 9;

FIG. 11 is a timing chart illustrating operation time of each componentof the image forming apparatus of FIG. 1;

FIG. 12 is a graph illustrating latent image diameters formed by theimage forming methods of a reference example and the present invention;

FIG. 13 is a schematic diagram illustrating an example of an exposuremethod of the reference example;

FIG. 14 is a schematic diagram illustrating an example of the imageforming method;

FIG. 15 is a schematic diagram illustrating another example of the imageforming method;

FIG. 16 is a schematic diagram illustrating a still further example ofthe image forming method;

FIG. 17 is a graph illustrating spatial frequency characteristicsaccording to a difference of the exposure methods;

FIG. 18 is a graph illustrating a relationship between a latent imagecircle diameter and a beam spot diameter;

FIG. 19 is a graph illustrating a measurement result of an MTF in alongitudinal direction;

FIGS. 20A to 20D are schematic diagrams illustrating examples ofexposure patterns of line images of a first exposure method according tothe present invention;

FIG. 21 is a schematic diagrams illustrating examples of the light powervalues of the exposure patterns of the line images in FIGS. 20A to 20D;

FIG. 22 is a graph illustrating electric field intensity distributionsof latent images of the exposure patterns of FIGS. 20A to 20D;

FIGS. 23A to 23D are schematic diagrams illustrating examples of theexposure patterns of the dot images of the first exposure method;

FIGS. 24A to 24D are schematic diagrams illustrating other examples ofthe exposure patterns of the dot images of the first exposure method;

FIGS. 25A to 25C are schematic diagrams illustrating still otherexamples of the exposure patterns of the dot images of the firstexposure method;

FIGS. 26A to 26D are schematic diagrams illustrating examples of theexposure patterns of images having a bent portion of the first exposuremethod;

FIG. 27 is a flowchart of an exposure method according to an embodiment;

FIGS. 28A to 28D are schematic diagrams illustrating examples ofexposure patterns of dot images of a second exposure method;

FIGS. 29A to 29D are schematic diagrams illustrating examples ofaddition processes of light power values of exposure patterns of a thirdexposure method;

FIGS. 30A and 30B are schematic diagrams illustrating other examples ofthe addition processes of the light power values of the exposurepatterns of the third exposure method;

FIG. 31 is a block diagram illustrating the third exposure method;

FIGS. 32A and 32B are schematic diagrams illustrating exposure patternsof character images according to the exposure method of the embodiment;

FIGS. 33A and 33B are schematic diagrams illustrating exposure patternsof outline character images according to the exposure method of theembodiment;

FIG. 34 is a schematic diagram illustrating a process mode determiningprocess in the first example of the image forming method according tothe present invention;

FIGS. 35A and 35B are schematic diagrams illustrating light power valuesof a high power exposure pixel group and a non-exposure pixel groupadjacent to a boundary pixel group in the first example;

FIG. 36 is a graph illustrating a latent image electric field intensitydistribution of an exposure pattern in the first example;

FIG. 37 is an enlarged diagram of a graph illustrating the latent imageelectric field intensity distribution of FIG. 36;

FIG. 38 is a flowchart illustrating processes of an exposing method inthe first example;

FIGS. 39A to 39C are schematic diagrams illustrating a relationshipamong image data, image identification information, and an exposingprocess in the case where an image pattern is a character-line image inthe second example of the image forming method according to the presentinvention;

FIGS. 40A to 40C are schematic diagrams illustrating a relationshipamong image data, image identification information, and an exposingprocess in the case where an image pattern is an outlined character-lineimage in the second example;

FIGS. 41A to 41C are schematic diagrams illustrating a relationshipamong image data, image identification information, and an exposingprocess in the case where an image pattern is a dither-processed imagein the second example;

FIGS. 42A to 42F are schematic diagrams illustrating an example of areference pixel of acquiring image identification information in a thirdexample of the image forming method according to the present invention;

FIG. 43 is a flowchart illustrating processes of an exposing methodaccording to a fourth example of the image forming method according tothe present invention;

FIG. 44 is a central cross-sectional view illustrating an example of anelectrostatic latent image measurement device;

FIG. 45 is a schematic diagram illustrating a relationship between anacceleration voltage and charging; and

FIG. 46 is a graph illustrating a relationship between the accelerationvoltage and a charge potential.

The accompanying drawings are intended to depict exemplary embodimentsof the present invention and should not be interpreted to limit thescope thereof. Identical or similar reference numerals designateidentical or similar components throughout the various drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentinvention.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

In describing preferred embodiments illustrated in the drawings,specific terminology may be employed for the sake of clarity. However,the disclosure of this patent specification is not intended to belimited to the specific terminology so selected, and it is to beunderstood that each specific element includes all technical equivalentsthat have the same function, operate in a similar manner, and achieve asimilar result.

An embodiment of the present invention will be described in detail belowwith reference to the drawings.

An embodiment has an object to provide an image forming method capableof forming a high-quality image by an image pattern including an imageportion composed of a plurality of pixels and a non-image portion.

Image Forming Apparatus

First, an image forming apparatus according to the present inventionwill be described.

FIG. 1 is a central cross-sectional view illustrating an embodiment ofan image forming apparatus according to the present invention. In thefigure, a schematic configuration of a laser printer 1000 is illustratedas the image forming apparatus according to the present invention.

The laser printer 1000 is constructed such that devices for performingan electrophotography process of charging, exposing, developing,transfer, and cleaning are arranged around a photoconductor drum 1030 inthis order along a rotation direction of the photoconductor drum 1030.More specifically, the laser printer includes a charging device 1031performing a charging process, an optical scanning device 1010performing an exposure process, a developing device 1032 performing adeveloping process, a transfer device 1033 performing a transferprocess, and a cleaning unit 1035 performing a cleaning process. Here, aneutralizing unit 1034 is disposed between the transfer device 1033 andthe cleaning unit 1035.

The developing device 1032 includes a toner cartridge 1036 and adeveloping roller (not illustrated) which adheres toner supplied fromthe toner cartridge 1036 on a surface of the photoconductor drum 1030and visualizes a latent image on the surface of the photoconductor drum1030 by the toner.

The transfer device 1033 transfers a toner image of the surface of thephotoconductor drum 1030 to a recording sheet 1040 drawn out from paperfeeding tray 1038 by a paper feeding roller 1037. A front end of therecording sheet 1040 is positioned by a registration roller 1039, andthe recording sheet is ejected through a fixing device 1041 to a paperejection tray 1043 by a paper ejection roller 1042 in synchronizationwith the toner image of the surface of the photoconductor drum 1030.

In addition, the laser printer 1000 includes a communication controldevice 1050 and a printer control device 1060.

Incidentally, the above-described components of the laser printer 1000are accommodated at predetermined positions inside a printer case 1044.

The communication control device 1050 controls bi-directionalcommunication with a host apparatus (for example, an informationprocessing apparatus such as a PC) via a network or the like.

The printer control device 1060 includes a Central Processing Unit (CPU)and a Read Only Memory (ROM), which are not illustrated. In addition,the printer control device 1060 includes a Random Access Memory (RAM)and an Analog/Digital (A/D) converter. Here, the printer control device1060 overall controls the components in response to requests from thehost apparatus and transmits image information of the host apparatus tothe optical scanning device 1010.

The ROM stores a program which is written in code readable by the CPUand various data used to execute the program.

The RAM is a temporary writable memory for a task of the CPU.

The A/D converter converts an analog signal into a digital signal.

The photoconductor drum 1030 is a latent image bearer of a cylindricalmember, and a photoconductor layer is formed on the surface thereof.That is, the surface of the photoconductor drum 1030 is a scanningsurface. In addition, the photoconductor drum 1030 is rotated by adriving mechanism (not illustrated) in the arrow direction in FIG. 1.

The charging device 1031 uniformly charges the surface of thephotoconductor drum 1030. Here, for example, a contact type chargingroller where a small amount of ozone is generated or a corona chargerusing corona discharge may be used for the charging device 1031.

FIG. 2 is a schematic diagram illustrating a corotron type chargingdevice of the image forming apparatus. In addition, FIG. 3 is aschematic diagram illustrating a scorotron type charging device of theimage forming apparatus. Here, the charging device 1031 may be thecorotron type charging device illustrated in FIG. 2, may be thescorotron type charging device illustrated in FIG. 3, or may be a rollertype charging device (not illustrated).

Returning to FIG. 1, the optical scanning device 1010 scans the surfaceof the photoconductor drum 1030 charged by the charging device 1031 withlight flux modulated based on the image information of the printercontrol device 1060 to perform exposure. The optical scanning device1010 forms the electrostatic latent image corresponding to the imageinformation on the surface of the photoconductor drum 1030.

The electrostatic latent image formed by the optical scanning device1010 is moved toward the developing device 1032 according to therotation of the photoconductor drum 1030. Incidentally, details of theoptical scanning device 1010 will be described later.

The toner cartridge 1036 contains the toner (developing agent). Thetoner is supplied from the toner cartridge 1036 to the developing device1032.

The developing device 1032 adheres the toner supplied from the tonercartridge 1036 to the latent image formed on the surface of thephotoconductor drum 1030 to develop the electrostatic latent image.Here, the image (hereinafter, referred to as a “toner image”) where thetoner is adhered is moved toward the transfer device 1033 according tothe rotation of the photoconductor drum 1030.

The paper feeding tray 1038 contains the recording sheet 1040. The paperfeeding roller 1037 is disposed adjacent to the paper feeding tray 1038.

The paper feeding roller 1037 draws the recording sheet 1040 out fromthe paper feeding tray 1038 one by one. The recording sheet 1040 isdrawn out from the paper feeding tray 1038 toward a gap between thephotoconductor drum 1030 and the transfer device 1033 in accordance withthe rotation of the photoconductor drum 1030.

The transfer device 1033 is applied with a voltage having a polarityopposite to the toner in order to electrically attract the toner of thesurface of the photoconductor drum 1030 to the recording sheet 1040. Dueto the voltage, the toner image of the surface of the photoconductordrum 1030 is transferred to the recording sheet 1040. The recordingsheet 1040 where the toner image is transferred is transported to thefixing device 1041.

In the fixing device 1041, heat and pressure are applied to therecording sheet 1040, so that the toner is fixed on the recording sheet1040. Here, the recording sheet 1040 where the toner is fixed is ejectedthrough the paper ejection roller 1042 to the paper ejection tray 1043to be sequentially stacked on the paper ejection tray 1043, so that aprint material is produced.

The neutralizing unit 1034 neutralizes the surface of the photoconductordrum 1030.

The cleaning unit 1035 removes the toner remaining on the surface of thephotoconductor drum 1030 (residual toner). The surface of thephotoconductor drum 1030 where the residual toner is removed is returnedto a position facing the charging device 1031.

In the image forming apparatus according to the present invention, theelectrostatic latent image is formed by the charging device, the opticalscanning device as an exposing device, the photoconductor, and the imageprocessing section for converting the image pattern into an opticaloutput.

The processes for obtaining the output image in the electrophotographymethod of a copier or a laser printer are as follows. That is, in theelectrophotography method, in the charging process, the photoconductoras one latent image bearer is uniformly charged. In addition, in theelectrophotography method, in the exposure process, the photoconductoris irradiated with light and charges are thereby partially escaped.Thereby, in the electrophotography method, the electrostatic latentimage can be formed on the photoconductor.

Configuration of Optical Scanning Device

Next, a configuration of the optical scanning device 1010 constitutingthe image forming apparatus will be described.

FIG. 4 is a schematic view illustrating an example of the opticalscanning device 1010. As illustrated in the figure, the optical scanningdevice 1010 includes a light source 11, a collimator lens 12, acylindrical lens 13, a folding mirror 14, a polygon mirror 15, and afirst scanning lens 21. In addition, the optical scanning device 1010further includes a second scanning lens 22, a folding mirror 24, asynchronization detection sensor 26, and a scanning control device (notillustrated).

Here, the optical scanning device 1010 is assembled at a predeterminedposition of an optical housing (not illustrated).

Incidentally, in the description hereinafter, the direction along thelongitudinal direction (rotation axis direction) of the photoconductordrum 1030 is called the Y axis direction of the XYZ three-dimensionalrectangular coordinate system, the direction along the rotation axis ofthe polygon mirror 15 is called the Z axis direction, and the directionperpendicular to the Y and Z axes is called the X axis direction.

In addition, in the description hereinafter, the direction correspondingto the main-scanning direction of each optical member is called the“main-scanning corresponding direction”, and the direction correspondingto the sub-scanning direction is called the “sub-scanning correspondingdirection”.

The light source 11 includes a plurality of light-emitting units (notillustrated) which are, for example, two-dimensionally arrayed. Here,when all the light-emitting units are orthogonally projected on avirtual line extending in the sub-scanning corresponding direction,light-emitting units are arrayed such that intervals between thelight-emitting units are equal.

Here, a semiconductor laser (Laser Diode: LD), a light emitting diode(Light Emitting Diode: LED), or the like may be used to construct thelight source 11.

FIG. 5 is a schematic diagram illustrating an example of the lightsource of the optical scanning device 1010. In the figure, a lightsource 11A as a multi-beam light source with a beam interval ds is asemiconductor laser array constructed by arraying four semiconductorlasers. In addition, the light source 11A is disposed to beperpendicular to the optical axis direction of the collimator lens 12.

FIG. 6 is a schematic view illustrating another example of the lightsource of the optical scanning device 1010. In the figure, a lightsource 11B is a vertical cavity surface emitting laser (VCSEL) having awavelength of, for example, 780 nm where light emitting points arearranged in a plane including the Y and Z axis directions.

The light source 11B has, for example, a total of twelve light emittingpoints, that is, three light emitting points in the horizontal direction(main-scanning direction, Y axis direction) and four light emittingpoints in the vertical direction (sub-scanning direction, Z axisdirection).

In addition, in the case where the light source 11B is applied to theoptical scanning device 1010, respective scan lines may be scanned withthree light emitting points arranged in the horizontal direction, sothat four scan lines in the vertical direction are simultaneouslyscanned.

Here, in the description hereinafter, a “light-emitting unit interval”denotes a distance between centers of two light-emitting units.

Returning to FIG. 4, the collimator lens 12 is disposed on the opticalpath of the light emitted from the light source 11 to control the lightto be parallel light or approximately parallel light.

The cylindrical lens 13 converges the light passing through thecollimator lens 12 only in the Z axis direction (sub-scanning direction)adjacent to a deflecting reflection plane of the polygon mirror 15.

The cylindrical lens 13 forms an image of the light emitted from thelight source 11 as a line image elongated in the main-scanning direction(Y axis direction) adjacent to a reflection plane of the folding mirror14.

The folding mirror 14 reflects the light having passed through thecylindrical lens 13 and imaged, toward the polygon mirror 15.

In addition, the optical system disposed on the optical path between thelight source 11 and the polygon mirror 15 is also called a pre-deflectoroptical system.

The polygon mirror 15 is a polygon mirror rotating around the rotationaxis perpendicular to the longitudinal direction (rotation axisdirection) of the photoconductor drum 1030. Here, each mirror plane ofthe polygon mirror 15 is a deflecting reflection plane.

A driving Integrated Circuit (IC) (not illustrated) applies appropriateclock to a motor unit (not illustrated), so that the polygon mirror 15is rotated at a desired constant speed.

The polygon mirror 15 is rotated at a constant speed in the arrowdirection by the motor unit, and a plurality of light beams reflected onthe deflecting reflection planes becomes respective deflecting beams tobe deflected at a constant angular velocity.

The first scanning lens 21, the second scanning lens 22, the foldingmirror 24, and the synchronization detection sensor 26 constitute ascanning optical system. The scanning optical system is disposed on theoptical path of the light deflected by the polygon mirror 15.

The first scanning lens 21 is disposed on the optical path of the lightdeflected by the polygon mirror 15.

The second scanning lens 22 is disposed on the optical path of the lightthrough the first scanning lens 21.

The folding mirror 24 is an elongated plane mirror and folds the opticalpath of the light through the second scanning lens 22 to the directiontoward the photoconductor drum 1030.

That is, the photoconductor drum 1030 is irradiated with the lightdeflected by the polygon mirror 15 through the first scanning lens 21and the second scanning lens 22, so that light spots are formed on thesurface of the photoconductor drum 1030.

The light spot of the surface of the photoconductor drum 1030 is movedalong the longitudinal direction of the photoconductor drum 1030according to the rotation of the polygon mirror 15. Here, the movementdirection of the light spot on the surface of the photoconductor drum1030 is the “main-scanning direction”, and the rotation direction of thephotoconductor drum 1030 is the “sub-scanning direction”. At least oneof the first scanning lens 21 and the second scanning lens 22 has an fθfunction that causes the light beam deflected at a constant angularspeed to move at a constant speed on the surface of the photoconductordrum 1030.

The synchronization detection sensor 26 receives the light from thepolygon mirror 15 and outputs a signal (photoelectric conversion signal)according to a received light amount to the scanning control device.Here, the output signal of the synchronization detection sensor 26 isalso called a “synchronization detection signal”.

As illustrated in FIG. 4, in the optical scanning device 1010, by thescanning using one deflecting reflection plane of the polygon mirror 15,a plurality of lines on the scanning surface of the photoconductor drum1030 is simultaneously scanned. A buffer memory inside the imageprocessing section controlling a light emitting signal of each lightemitting point stores print data for one line corresponding to eachlight emitting point.

The print data are read out for each deflecting reflection plane of thepolygon mirror 15, and a light beam is turned on and off on the scanline on the photoconductor drum 1030 as the latent image beareraccording to the print data, so that the electrostatic latent image isformed along the scan line.

FIG. 7 is a block diagram illustrating the image processing section ofthe image forming apparatus. As illustrated in the figure, the imageprocessing section includes an image processing unit (Image ProcessingUnit: IPU) 101, a controller unit 102, a memory unit 103, an opticalwriting output unit 104, and a scanner unit 105.

The controller unit 102 performs processes of rotation, repeating,collection, compression, decompression, and the like on the image dataand then, outputs the processed image data to the IPU again.

In the memory unit 103, a lookup table for storing various data isprepared.

The optical writing output unit 104 performs optical modulation of thelight source 11 according to the lighting data by a control driver andforms the electrostatic latent image on the photoconductor drum 1030.Here, the optical writing output unit 104 forms the electrostatic latentimage based on an input signal output from the later-described gradationprocessing unit. The formed electrostatic latent image causes thedeveloping device 1032, the transfer device 1033, and the like abovedescribed to form an image on the recording sheet.

The scanner unit 105 reads the image and generates image data such asRed, Green, and Blue (RGB) data based on the image.

FIG. 8 is a block diagram illustrating the image processing unit 101 ofthe image processing section. As illustrated in the figure, imageprocessing unit 101 includes a density conversion unit 101 a, a filterunit 101 b, a color correction unit 101 c, a selector unit 101 d, agradation correction unit 101 e, and a gradation processing unit 101 f.

The density conversion unit 101 a converts the RGB image data of thescanner unit 105 into the density data using the lookup table andoutputs the density data to the filter unit 101 b.

The filter unit 101 b performs image correction processes such as asmoothing process or an edge enhancing process on the density data inputfrom the density conversion unit 101 a and output the density data afterthe image correction processes to the color correction unit 101 c.

The color correction unit 101 c performs a color correction (masking)process.

Under the control of the image processing unit 101, the selector unit101 d selects any of Cyan (C), Magenta (M), Yellow (Y), and Key Plate(K) from the image data input from the color correction unit 101 c. Theselector unit 101 d outputs the data of selected C, Y, M, and K to thegradation correction unit 101 e.

The gradation correction unit 101 e stores the data of C, M, Y, and Kinput from the selector unit 101 d in advance. In the gradationcorrection unit 101 e, a γ curve from which linear characteristics areobtained is set for the input data.

The gradation processing unit 101 f performs a gradation process such asa dither process on the image data input from the gradation correctionunit 101 e and outputs the resulting signal to the optical writingoutput unit 104.

Light Source Driving Unit

Next, the light source driving unit of the image forming apparatusaccording to the present invention which performs the image formingmethod according to the present invention will be described.

FIG. 9 is a circuit diagram illustrating the light source driving unitconstituting the image forming apparatus of FIG. 1. As illustrated inthe figure, the light source driving unit 410 includes current sources201 to 204, switches SW1 to SW4, and a memory 205. In addition, thelight source driving unit 410 is connected to a light source modulationdata generating circuit (image processing circuit) 407.

In the image forming apparatus according to the present invention whichperforms the image forming method according to the present invention,the exposure is performed while changing the light power valuecorresponding to the position in the main scanning direction in theimage portion (corresponding to the time from the start of the exposureof the image portion). By the configuration illustrated in FIG. 9, thelight source driving unit 410 may simultaneously performs pulse widthmodulation and light intensity modulation (PWM+PW modulation) togenerate a light source driving current.

In general, a bias current (Ibi), a basic pattern current (Iop), andovershoot currents (Iov1 and Iov2) are added to generate a currentwaveform.

The current source 201 generates the overshoot current Iov1. Inaddition, the current source 202 generates the overshoot current Iov2.In addition, the current source 203 generates the basic pattern currentIop. In addition, the current source 204 generates the bias current Ibi.

Here, the current sources 201 to 204 are controlled by a current valuecontrol signal from the light source modulation data generating circuit407, so that the current value generated by the light source drivingunit 410 is determined.

The switches SW1 to SW4 are installed to correspond to the currentsources 201 to 204. The switches SW1 to SW4 are controlled by lightsource modulation signals OV1, OV2, OP, BI for the overshoot currentIov1, the overshoot current Iov2, the basic pattern current Iop, and thebias current Ibi from the light source modulation data generatingcircuit 407. The switches SW1 to SW4 control the flow of the currentsources 201 to 204 to generate a pattern of pulses generated by thelight source driving unit 410.

The memory 205 corresponds to a storage unit and stores informationrequired during a light source driving current generation period. Thelight source modulation data generating circuit 407 refers to theinformation of the memory 205.

According to the light source driving unit 410, since the light sourcemodulation signal obtained from the light source modulation data can beconverted into the current, the image forming apparatus according to thepresent embodiment can generate PM+PWM modulation which simultaneouslycontrols the light power and the lighting time.

FIG. 10 is a block diagram illustrating the light source driving controlunit of FIG. 9. As illustrated in the figure, the light source drivingcontrol unit 1019 includes a reference clock generating circuit 422 anda pixel clock generating circuit 425. In addition, the light sourcedriving control unit 1019 includes a light source modulation datagenerating circuit 407, a light source selecting circuit 414, a writetiming signal generating circuit 415, and a synchronization timingsignal generating circuit 417.

Incidentally, in FIG. 10, the arrows illustrate the representative flowsof signals and information, but the arrows do not illustrate all theconnection relationship between the respective blocks.

The reference clock generating circuit 422 generates a high frequencyclock signal which is used as a reference of the entire light sourcedriving control unit 1019.

The pixel clock generating circuit 425 mainly includes a Phase LockedLoop (PLL) circuit. The pixel clock generating circuit 425 generates apixel clock signal based on a synchronization signal s19 and ahigh-frequency clock signal of the reference clock generating circuit422.

Here, the pixel clock signal is configured such that the frequency ofthe pixel clock signal is the same as the frequency of thehigh-frequency clock signal and the phase of the pixel clock signal iscoincident with the synchronization signal s19.

Therefore, the pixel clock generating circuit 425 synchronizes the imagedata with the pixel clock signal to control the writing position foreach scanning.

Here, the generated pixel clock signal is supplied as a kind of thedriving information to the light source driving unit 410 and is alsosupplied to the light source modulation data generating circuit 407. Thepixel clock signal supplied to the light source modulation datagenerating circuit 407 is used as a clock signal for writing data s16.

The light source selecting circuit 414 is a circuit used in the casewhere a plurality of the light sources is used and outputs a signaldesignating the selected light-emitting unit. The output signal s14 ofthe light source selecting circuit 414 is supplied as a kind of thedriving information to the light source driving unit 410.

FIG. 11 is a timing chart illustrating operation time of each componentof the image forming apparatus of FIG. 1. In the figure, s19 denotes anoutput signal (synchronization signal) of the synchronization detectionsensor 26. In addition, s15 denotes an output signal (LGATE signal) ofthe write timing signal generating circuit 415. In addition, s14 denotesan output signal of the light source selecting circuit 414. In addition,s16 denotes the writing data as an output signal of the light sourcemodulation data generating circuit 407. In addition, t1 denotes the timebetween the end of the synchronization signal S19 and the start ofwriting. In addition, t2 denotes the writing time.

The light source modulation data generating circuit 407 corresponds tothe light source driving unit of the image forming apparatus accordingto the present invention. That is, the light source modulation datagenerating circuit 407 generates the writing data s16 for eachlight-emitting unit based on the image information of the imageprocessing unit (IPU) or the like. The writing data s16 are supplied asa kind of the driving information to the light source driving unit 410according to the timing of the pixel clock signal.

Here, in order to form the latent image according to the exposure methodaccording to the embodiment, the light source modulation data generatingcircuit 407 converts the image data into an exposure pattern accordingto the PM+PWM signal based on the image pattern information or taginformation from the image processing unit.

Image Forming Method

Next, the exposure method in the embodiment of the image forming methodaccording to the present invention will be described.

In the image forming method according to the embodiment, the opticaloutput waveform used for the latent image formation is a waveform forexposing the photoconductor for a predetermined time with the lightpower value required to obtain a target image density in the imageportion including the line image or the solid image.

In addition, the image portion is composed of a plurality of pixels andis a portion for adhering toner to form an image in the image pattern.In addition, the non-image portion is a portion where no toner isadhered in the image pattern and no image is formed.

In the description hereinafter, the image density as a target is calleda “target image density”. In addition, in the description hereinafter, apredetermined light power value required to obtain the target imagedensity is called a “target exposure output value”. In addition, in thedescription hereinafter, a predetermined time for exposing the entirepixels of the image portion with the target exposure output value toobtain the target image density is called a “target exposure time”.

In addition, in the description hereinafter, an exposure method ofexposing for the target exposure time with the target exposure outputvalue is called “standard exposure”. In addition, in the embodiment, thesolid image denotes an image portion having an area larger than an areaof a line image.

In addition, in the description hereinafter, the exposing thephotoconductor with the light power value (first light power value)higher than the target exposure output value for the exposure timeshorter than the target exposure time is called “time concentrationexposure”.

In addition, in the description hereinafter, the time concentrationexposure may also be called TC (Time Concentration) exposure.

FIG. 12 is a graph illustrating a latent image diameter formed by theimage forming methods according to the reference example and the presentinvention. The figure illustrates results of simulation of 2-dot latentimage charge distributions in the case where the dot density is 1200 dpiaccording to the exposure method of the reference example where thestandard exposure is performed and the exposure method of the embodimentwhere the time concentration exposure is performed. Here, in the timeconcentration exposure, the light power value of the image pixel to 400%of the target exposure output value is set to perform the exposure.

The latent image charge distribution illustrated in FIG. 12 illustratesthat the latent image diameter in the time concentration exposure of thebeam spot diameter 70×90 μm and the latent image diameter in thestandard exposure of the beam spot diameter 55×55 μm are equivalent toeach other. That is, according to the embodiment, the effect equivalentto the effect of the reduction of the beam spot diameter in the standardexposure can be obtained by using the time concentration exposure.

FIG. 13 is a schematic diagram illustrating an example of the exposuremethod in the reference example. As illustrated in the figure, theexposure method (hereinafter, referred to as an “exposure method 1”)according to the standard exposure of the reference example is awaveform for exposing the photoconductor for the target exposure timewith the target exposure output value as described above with respect tothe 1-dot image portion including the line image or the solid image.Here, the target exposure output value is set to 100% of the light powervalue, and the target exposure time is set to a duty ratio of 100%.

FIG. 14 is a schematic diagram illustrating an example of the imageforming method according to the present invention. As illustrated in thefigure, in the exposure method (hereinafter, referred to as an “exposuremethod 2”) according to the time concentration exposure according to theembodiment, the photoconductor is exposed with the target exposureoutput value being set to 200% of the light power value and with thetarget exposure time being set to a duty ratio of 50%. Here, when thewidth of the image portion is set to one, the width of the exposingsection is 4/8 pixels.

FIG. 15 is a schematic diagram illustrating another example of the imageforming method according to the present invention. As illustrated in thefigure, in the exposure method (hereinafter, referred to as an “exposuremethod 3”) according to the time concentration exposure according to theembodiment, the photoconductor is exposed with the target exposureoutput value being set to 400% of the light power value and with thetarget exposure time being set to a duty ratio of 25%. Here, if thewidth of the image portion is set to one, the width of the exposingsection is 2/8 pixels.

FIG. 16 is a schematic diagram illustrating still another example of theimage forming method according to the present invention. As illustratedin the figure, in the exposure method (hereinafter, referred to as an“exposure method 4”) according to the time concentration exposureaccording to the embodiment, the photoconductor is exposed with targetexposure output value being set to 800% of the light power value andwith the target exposure time being set to a duty ratio of 12.5%. Here,when the width of the image portion is set to one, the width of theexposing section is ⅛ pixels.

In the above-described exposure methods 2 to 4, the pulse widths aresmaller than the pulse widths of the exposure method 1. That is, in theexposure methods 2 to 4, the formed latent image becomes small when theexposure is performed with the same light amount as the light amount ofthe exposure method 1, and therefore the light amounts are controlledaccording to the pulse widths so that the integrated light amountsduring the latent image formation period are equivalent to each other.

That is, in the exposure methods 2 to 4 according to the timeconcentration exposure, the exposure is performed with a small pulsewidth and a strong light intensity in comparison with the exposuremethod 1 according to the standard exposure.

Incidentally, in the description above, in the exposure methods 2 to 4,the light power value is set so that the integrated light amount isconstant. However, in the image forming method according to the presentinvention, the configuration is not limited to this configuration.

In the embodiment, in the case where the beam spot diameter used for theexposure is 70 μm (main-scanning direction)×90 μm (sub-scanningdirection), when the exposure is performed with a pulse width smallerthan 1 pixel as described above, latent image formation capability isevaluated by the later-described evaluation method. Thereby, in theembodiment, the exposure method capable of improving the latent imageresolution without changing the beam spot diameter used for the exposureis examined.

FIG. 17 is a graph illustrating spatial frequency characteristicsaccording to a difference of the exposure methods. As illustrated in thefigure, in the exposure methods 2 to 4, a latent image ModulationTransfer Function (MTF) shows a high value up to a high frequency bandin comparison with the exposure method 1.

The graph of FIG. 17 illustrates that, in the exposure methods 2 to 4,the smaller-diameter latent image can be stably formed in comparisonwith the exposure method 1. Particularly, it is illustrated that, amongthe exposure methods 2 to 4, the exposure method 4 where the pulse widthis smallest is appropriate for the stable formation of thesmall-diameter latent image.

In addition, the graph of FIG. 1 illustrates that, in the exposuremethods 2 to 4, since the exposure is performed with the small pulsewidth and the strong light intensity, the latent image resolution isimproved in comparison with the exposure method 1. That is, it isillustrated that, according to the exposure methods 2 to 4 used for theimage forming method according to the present invention, thesmall-diameter latent image can be stably formed in comparison with theexposure method 1 used for the image forming method of the related art.

FIG. 18 is a graph illustrating a relationship between a latent imagecircle diameter and a beam spot diameter (beam size). The figureillustrates a relationship between a latent image circle correspondingdiameter where a latent image MTF representing a latent image dotdensity is 80% and the beam spot diameter. As illustrated in the figure,the latent image resolution and the beam spot diameter change inproportion to each other.

In the image forming method according to the present invention, in thecase where the stability of a latent image in a high-frequency region,that is, a latent image having a small diameter is emphasized, theexposure method according to the time concentration exposure has asuperiority to the case where exposure is performed with a small beamspot diameter according to the exposure method of the related art. Here,the optimal beam spot diameter according to the difference of the outputimages is determined by the latent image MTF at the maximum spatialfrequency required as the output image.

It should be further noted that the width of the latent image electricvector is narrow in comparison with other means, which means that thelatent image electric vector is increased as well as the resolution isimproved.

In addition, in the image forming method according to the presentinvention, unlike the case where the light source is controlled throughthe power modulation or the pulse width modulation to perform theexposure, the integrated light amount is equal to the case where theexposure is performed with the target exposure output value. For thisreason, in the image forming method according to the present invention,the adhesion amount of toner or the total image density is essentiallynot different from the case where the exposure is performed with thetarget exposure output value.

FIG. 19 is a graph illustrating a measurement result of a latent imageMTF in a longitudinal direction. In the figure, the first light powervalue illustrated in FIG. 15 represents a value obtained by actuallypaper-outputting a vertical line image exposed with a value of 400% of atarget exposure output value and MTF-analyzing. The figure illustratesthat, with respect to any line width, the MTF of the exposure methodaccording to the embodiment is higher than the MTF of the exposuremethod of the related art.

Particularly, the effect that the MTF increases as the frequencyincreases is remarkable.

As described above, in the case of the PM modulation where theirradiation can be performed with a light power value P1 higher than atarget exposure output value P0 at the time of forming a solid imagedensity, a ratio (TCR) of light power values is defined as TCR=P1/P0.

In this case, in the exposure method according to the embodiment, awidth of a longitudinal line is compressed to 1/TCR, and the exposure isperformed with the light power value higher than the target exposureoutput value at the time of the solid image density. Thereby, accordingto the exposure method according to the embodiment, an image having ahigh MTF resolution can be formed.

In the exposure method according to the embodiment, strong light isconcentrated to expose the narrow range of the image portion where theimage is to be formed in the image pattern. Thereby, in the exposuremethod according to the embodiment, the fidelity of the micro-sizedoutput image pattern smaller than the beam diameter size (the influenceof the size of the beam diameter cannot be ignored) can be improved, andthe image pattern can be adjusted with a desired image density.

That is, according to the exposure method according to the embodiment,the output image compatibly realizing the formation of the micro-sizedimage pattern and the desired image density can be formed.

In addition, the exposure method according to the embodiment can beeasily applied to any image pattern without performing any particularprocess such as edge detection or character information recognition.

Therefore, according to the exposure method according to the embodiment,even in the case where object information cannot be obtained from acomputer when the image data are converted into the light sourcemodulation data, the image pattern can be generated.

In addition, according to the exposure method according to theembodiment, the output image compatibly realizing the formation of themicro-sized image pattern and the desired image density can be formedwithout associating the image data and the light source modulation datafor each character.

In addition, the exposure method according to the embodiment uses thePM+PWM modulation which is a combination of the Phase Modulation (PM)and the Pulse Width Modulation (PWM). In addition, according to theexposure method according to the embodiment, the integrated light amountof the image pattern during the exposing period may be the same value asthe standard exposure by using the time concentration exposure where themaximum light power is intentionally set to be strong.

Here, according to the exposure method according to the embodiment, theresolution of the image pattern can be improved by forming a depthlatent image without changing the image density of the image pattern.

In the exposure method according to the embodiment, the light powervalue is set such that the one or more pixels (pixel groups) inside theimage portion existing at the boundary between the image portion and thenon-image portion included in the image pattern become non-exposurepixels. Here, the group that is not exposed inside the image portionexisting at the boundary between the image portion and the non-imageportion included in the image pattern is called a non-exposure pixelgroup. In addition, in the exposure method according to the embodiment,the exposure is performed with the light power value obtained by addingthe light power value for the pixel group adjacent to the non-exposurepixel group (in the vicinity of the non-exposure pixel group) and thelight power value for the non-exposure pixel group.

Thereby, according to the exposure method according to the embodiment,the high-quality image pattern can be formed.

Example of Formation of Line Image

Next, an example of formation of a line image by the exposure methodaccording to the embodiment will be described. In addition, in thedescription hereinafter, in the figure, the Y axis direction(main-scanning direction) is set to the horizontal direction, and the Zaxis direction (sub-scanning direction) is set to the verticaldirection.

FIGS. 20A to 20D are schematic diagrams illustrating examples of theexposure patterns of line images in the first exposure method accordingto the present invention. FIG. 20A illustrates an exposure pattern 400 aof a line image according to the standard exposure.

In addition, FIG. 20B illustrates an exposure pattern 400 b of a lineimage where one dot at the boundary between the image portion and thenon-image portion 412 is set to a high power exposure pixel group 443.In addition, FIG. 20C illustrates an exposure pattern 400 c of a lineimage where two dots at the boundary between the image portion and thenon-image portion 412 are set to a high power exposure pixel group 443.In addition, FIG. 20D illustrates an exposure pattern 400 d of a lineimage where three dots at the boundary between the image portion and thenon-image portion 412 are set to a high power exposure pixel group 443.

In all the exposure patterns 400 a, 400 b, 400 c, and 400 d illustratedin FIGS. 20A, 20B, 20C, and 20D, the minimum pixel is 4800 dpi, and thespatial frequency is 6 c/mm. In the exposure patterns 400 a, 400 b, 400c, and 400 d, a bold longitudinal line (line in the Z axis direction) isformed every 8×8 dots (corresponding to 600 dpi).

That is, the exposure pattern 400 a illustrated in FIG. 20A includes anexposure portion (matching with the image portion) 411 and a non-imageportion 412 composed of two vertical lines having 600 dpi. Here, thesize of one pixel is about 5 μm.

In the exposure method according to the embodiment, the light powervalue is set such that, in the exposure pattern 400 b, the pixel groups(for example, a plurality of images where one pixel in the Y axisdirection is arranged in one row in the Z axis direction) existing atthe boundary between the image portion and the non-image portion 412become the non-exposure portion 441. Here, also in the exampleshereinafter, the non-exposure portion 441 corresponds to theabove-described non-exposure pixel group. In addition, in the exposuremethod according to the embodiment, the pixel groups (for example, aplurality of the pixel groups where one pixel in the Y axis direction isarranged in one row in the Z axis direction) existing at the boundarybetween the exposure portion 411 and the non-exposure portion 441 areset as the high power exposure pixel group 443.

In addition, in the exposure method according to the embodiment, when amagnification ratio of the time concentration exposure to the standardexposure is 2, the high power exposure pixel group 443 is exposed withtwice the light power. At this time, since the non-exposure portion 441is not exposed, the integrated light amount of the entire exposurepattern 400 b is the same as the integrated light amount of the exposurepattern 400 a.

In addition, in the exposure method according to the embodiment, thenumber of pixels of the non-exposure portion 441 and the high powerexposure pixel group 443 may be set to an arbitrary number of pixels inthe main-scanning direction or the sub-scanning direction.

The exposure pattern 400 c is set such that the non-exposure portion 441and the high power exposure pixel group 443 have a width of two pixelsin the Y axis direction. In addition, the exposure pattern 400 d is setsuch that the non-exposure portion 441 and the high power exposure pixelgroup 443 have a width of three pixels in the Y axis direction.

FIG. 21 is a schematic diagrams illustrating examples of the light powervalues of the exposure patterns of the line images in FIGS. 20A to 20D.Here, FIG. 21 illustrates, at (a), the light power value of the exposurepattern 400 a in FIG. 20A. In addition, FIG. 21 illustrates, at (b), thelight power value of the exposure pattern 400 b in FIG. 20B. Inaddition, FIG. 21 illustrates, at (c), superposition of the light powervalues illustrated at (a) and (b) in FIG. 21.

In FIG. 21, the horizontal axis denotes the dots in the Y axis directionin FIG. 10, and the vertical axis denotes the light power values of therespective dots. In addition, in the figure, numerical values in thedots denote multiples of the light power value.

As illustrated at (a) in FIG. 21, in the exposure pattern 400 aaccording to the standard exposure, the multiples of the light powervalues of all the dots in the Y axis direction are one, and the exposureis performed with the uniform light power value.

On the other hand, as illustrated at (b) in FIG. 21, in the exposurepattern 400 b according to the time concentration exposure, since thepixels (boundary pixels) existing at the boundary between the imageportion and the non-image portion become the non-exposure portions, themultiples of the light power values of the non-exposure portions arezero (light power values are zero). In addition, in the exposure pattern400 b, since the pixels existing at the boundary between the imageportion and the non-exposure portion become the high power exposurepixel groups, the multiples of the light power values of the high powerexposure pixel groups are two.

In addition, as illustrated at (c) in FIG. 21, in comparison with thewaveform (a) of the light power value according to the standard exposureand the waveform (b) of the light power value according to the timeconcentration exposure, both ends portions of the waveform (a) accordingto the standard exposure become the non-exposure portions in thewaveform (b) according to the time concentration exposure.

Next, the light power values of the non-exposure portion in the waveform(a) according to the standard exposure is added to the light powervalues of the high power exposure pixel groups corresponding to the bothends portions of the waveform (b) according to the time concentrationexposure. That is, the high power exposure pixel group corresponds to,so to speak, a process of folding the light power value inwards toincrease the light power value of the end portion of the image pattern.

FIG. 22 is a graph illustrating electric field intensity distributionsof latent images of the exposure patterns of FIGS. 20A to 20D. Thefigure illustrates the electric field intensity distribution of latentimage of the image portion according to the standard exposure and theelectric field intensity distribution of latent image of the imageportion according to the time concentration exposure where replacementof the non-exposure pixel group and the high power exposure pixel groupfor two dots is performed.

As illustrated in FIG. 22, in comparison with the electric fieldintensity distribution of latent image according to the standardexposure and the electric field intensity distribution of latent imageaccording to the time concentration exposure, it is found out that thetime concentration exposure is useful for the image formation becausethe width of the peak portion of the electric field intensity is smalland the slope of change of the electric field intensity is large (edgeis steep).

Example of Formation of Dot Image

The exposure method according to the embodiment described above is notlimited to the above-described line image, but the method may be appliedto a dot image or a character image.

FIGS. 23A to 23D are schematic diagrams illustrating examples ofexposure patterns of dot images in the first exposure method. FIG. 23Aillustrates an exposure pattern 500 a of a dot image according to thestandard exposure. In FIG. 23A, in the exposure pattern 500 a, theentire exposure portion 501 is an image portion which is exposed with auniform light power value, and the outside of the frame of the imageportion is a non-image portion.

Here, as illustrated in FIG. 23A, the exposure pattern 500 a is a squareof 24 dots of 4800 dpi (three dots when converted to 600 dpi).

In addition, FIG. 23B illustrates an exposure pattern 500 b of a dotimage where two dots existing at the boundary with respect to thenon-exposure portion 541 are set to high power exposure pixel groups 543a and 543 b. In addition, FIG. 23C illustrates another example of anexposure pattern 500 c of a line image where two dots existing at theboundary with respect to the non-exposure portion 541 are set to thehigh power exposure pixel groups 543 a, 543 b, and 543 c. In addition,FIG. 23D illustrates still another example of an exposure pattern 500 dof a line image where two dots existing at the boundary with respect tothe non-exposure portion 541 are set to the high power exposure pixelgroups 543 a, 543 b, and 543 c.

As illustrated in FIG. 23B, in the exposure pattern 500 b, among thepixels of the image portion, the two dots of the both ends in the Y andZ axis directions are set to the non-exposure portion 541. In addition,the high power exposure pixel groups 543 a and 543 b of the exposurepattern 500 b are added with the light power values of the pixelscorresponding to the two dots in the Y and Z directions adjacent to theexposure portion 501 and the non-image portion.

In this case, since the light power values of the high power exposurepixel groups 543 b of the four corners of the exposure portion are addedwith the light power values of both the non-exposure portions 541 in theY and Z axis direction, the light power values of the high powerexposure pixel groups are four times the light power value of theexposure portion 501 (twice the light power value of the high powerexposure pixel group 543 a).

However, in the image forming apparatus performing the exposure methodaccording to the embodiment, in some cases, it may be difficult to setthe light power value to four times the light power value of thestandard exposure.

In this case, the maximum value (hereinafter, referred to as a “maximumlight power value”) of the light power value is set in advance, and thelight power value can be limited within a range up to the maximum lightpower value. For example, the maximum light power value may be set to bethree times or twice the light power value of the standard exposure.

In the case of setting the maximum light power value in this manner, theintegrated light amount is slightly changed in comparison with the caseof the standard exposure. However, it can be said that since theoccupation ratio of the pixels influenced by the maximum light powervalue to the entire pattern is small, the image density of the formedimage is not greatly changed.

However, in the case where the dot size of the image pattern is smalland the number of pixels used for the high power exposure pixel group islarge, the occupation ratio of the pixels influenced by the maximumlight power value cannot be ignored, the appropriateness thereof may beselected based on the balance of the cost for implementation and theimage quality required.

FIG. 23C illustrates an exposure pattern 500 c in the case where themaximum light power value is set to be three times the light power valueof the standard exposure. As illustrated in the figure, in the exposurepattern 500 c, the light power value of the high power exposure pixelgroup 543 b which is to be exposed with four times the light power valueof the standard exposure in the original case is set to be three timesthe light power value of the standard exposure.

Here, in the exposure pattern 500 c, in order to equalize the integratedlight amount with the integrated light amount of the standard exposure,the light power value is dispersed such that a portion of the pixelswhich are adjacent to the high power exposure pixel group 543 b and donot reach the maximum light power value, are placed in the exposureportion 501 as the high power exposure pixel group 543 c.

FIG. 23D illustrates an exposure pattern 500 d in the case where themaximum light power value is set to be twice the light power value ofthe standard exposure. As illustrated in the figure, in the exposurepattern 500 d, the light power value of the high power exposure pixelgroup 543 b which is to be exposed with four times the light power valueof the standard exposure in the original case is set to be twice thelight power value of the standard exposure as in the high power exposurepixel group 543 a.

Here, in the exposure pattern 500 d, in order to equalize the integratedlight amount with the integrated light amount of the standard exposure,the light power value is dispersed to the exposure portion 501 a in thenon-exposure portion 541 which are pixels that are adjacent to the highpower exposure pixel group 543 b and do not reach the maximum lightpower value, as well as the high power exposure pixel group 543 c.

FIGS. 24A to 24D are schematic diagrams illustrating other examples ofthe exposure patterns of the dot images in the first exposure method.FIG. 24A illustrates an exposure pattern 600 a of a dot image accordingto the standard exposure. In FIG. 24A, in the exposure pattern 600 a,the entire exposure portion 601 is an image portion which is exposedwith a uniform light power value, and the outside of the frame of theimage portion is a non-image portion.

Here, as illustrated in FIG. 24A, the exposure pattern 600 a is a squareof 12 dots having 4800 dpi.

In addition, FIG. 24B illustrates an exposure pattern 600 b of a dotimage where two dots existing at the boundary with respect to thenon-exposure portion 641 are set to high power exposure pixel groups 643a and 643 b. In addition, FIG. 24C illustrates an example of an exposurepattern 600 c of a line image where the entire exposure portionincluding two dots existing at the boundary with respect to thenon-exposure portion 641 are set to the high power exposure pixel group643 a. In addition, FIG. 24D illustrates still another example of anexposure pattern 600 d of a line image where two dots existing at theboundary with respect to the non-exposure portion 641 are set to thehigh power exposure pixel group 643 a.

As illustrated in FIG. 24B, in the exposure pattern 600 b, among thepixels of the image portion, the two dots of the both ends in the Y andZ axis directions are set to the non-exposure portion 641. In addition,the high power exposure pixel groups 643 a and 643 b of the exposurepattern 600 b are added with the light power values of the pixelscorresponding to the two dots in the Y and Z directions adjacent to theexposure portion 601 and the non-image portion.

In this case, since the light power values of the high power exposurepixel group 643 b of the four corners of the exposure portion are addedwith the light power values of both the non-exposure portions 641 in theY and Z axis directions, the light power values of the high powerexposure pixel group are four times the light power value of theexposure portion 601 (twice the light power value of the high powerexposure pixel group 643 a).

FIG. 24C illustrates an exposure pattern 600 c in the case where themaximum light power value is set to be three times the light power valueof the standard exposure. As illustrated in the figure, in the exposurepattern 600 c, the light power value of the high power exposure pixelgroup 643 b which is to be exposed with four times the light power valueof the standard exposure in the case of the exposure pattern 600 b isset to be three times the light power value of the standard exposure.

Here, in the exposure pattern 600 c, in order to equalize the integratedlight amount with the integrated light amount of the standard exposure,the light power value is dispersed to the high power exposure pixelgroup 643 a. That is, in the exposure pattern 600 c, the entire exposureportion is set to the high power exposure pixel group 643 a which isexposed with three times the light power value of the standard exposure.

FIG. 24D illustrates an exposure pattern 600 d in the case where themaximum light power value is to be twice the light power value of thestandard exposure. As illustrated in the figure, in the exposure pattern600 d, the entire exposure portion is set to the high power exposurepixel group 643 a.

Here, in the exposure pattern 600 d, in order to equalize the integratedlight amount with the integrated light amount of the standard exposure,the light power value is dispersed to the exposure portion 601 a in thenon-exposure portion 641 which are the pixels which are adjacent to thehigh power exposure pixel group 643 a and do not reach the maximum lightpower value.

FIGS. 25A to 25C are schematic diagrams illustrating still otherexamples of exposure patterns of dot images in the first exposuremethod. FIG. 25A illustrates an exposure pattern 700 a of a dot imageaccording to the standard exposure. In FIG. 25A, in the exposure pattern700 a, the entire exposure portion 701 is an image portion which isexposed with a uniform light power value, and the outside of the frameof the image portion is a non-image portion.

Here, as illustrated in FIG. 25A, the exposure pattern 700 a is a squareof eight dots of 4800 dpi (one dot when converted to 600 dpi).

In addition, FIG. 25B illustrates an exposure pattern 700 b of a dotimage where two dots existing at the boundary with respect to thenon-exposure portion 741 are set to be a high power exposure pixel group743. In addition, FIG. 15C illustrates another example of an exposurepattern 700 c of a line image where the entire exposure portionincluding two dots existing at the boundary with respect to thenon-exposure portion 741 is set to be a high power exposure pixel group743 a.

As illustrated in FIG. 25B, in the exposure pattern 700 b, among thepixels of the image portion, the two dots of the both ends in the Y andZ axis directions are set to the non-exposure portion 741. That is, thehigh power exposure pixel group 743 of the exposure pattern 700 b isadded with the light power values of the pixels corresponding to the twodots in the Y and Z axis directions of the non-exposure portion 741 overthe entire exposure portion.

In this case, since the light power value of the high power exposurepixel group 743 is added with the light power values of the both ends ofthe non-exposure portion 741 in the Y and Z axis directions, the lightpower value of the high power exposure pixel group is four times thelight power value of the exposure portion 701 of the exposure pattern700 a.

FIG. 25C illustrates an exposure pattern 700 c in the case where themaximum light power value is set to be twice the light power value ofthe standard exposure. As illustrated in the figure, in the exposurepattern 700 c, the light power values of the high power exposure pixelgroups 743 a and 743 b which are to be exposed with four times the lightpower value of the standard exposure in the exposure pattern 700 b areset to be twice the light power value of the standard exposure.

Here, in the exposure pattern 700 c, in order to equalize the integratedlight amount with the integrated light amount of the standard exposure,the light power value is dispersed to the high power exposure pixelgroup 743 b. That is, in the exposure pattern 600 c, in order toequalize the integrated light amount with the integrated light amount ofthe standard exposure, the light power value is dispersed to the highpower exposure pixel group 743 b in the non-exposure portion 741 whichare the pixels which are adjacent to the high power exposure pixel group743 a and do not reach the maximum light power value.

Example of Image Having Bent Portion

FIGS. 26A to 26D are schematic diagrams illustrating examples ofexposure patterns of images having a bent portion in the first exposuremethod. These figures illustrate examples of pinched shaped imagepatterns having a bent portion which is assumed as a more general imagepattern. In the case of such an image pattern, one image pattern may bedivided into two or more image patterns for processing. The figureillustrates an example where the light power value of one image patternis uniformly distributed to two places to set three light power values.

FIG. 26A illustrates an exposure pattern 800 a of an image according tothe standard exposure. In FIG. 26A, in the exposure pattern 800 a, theentire exposure portion 801 is an image portion which is exposed with auniform light power value, and the outside of the frame of the imageportion is a non-image portion 802.

Here, as illustrated in FIG. 26A, the exposure pattern 800 a is a squareof 24 dots of 4800 dpi (three dots when converted to 600 dpi).

In addition, FIG. 26B illustrates an exposure pattern 800 b of a dotimage where two dots existing at the boundary with respect to thenon-exposure portion 841 are set to be high power exposure pixel groups843 a and 843 b. In addition, FIG. 26C illustrates another example of anexposure pattern 800 c of a line image where two dots existing at theboundary with respect to the non-exposure portion 841 are set to be thehigh power exposure pixel groups 843 a and 843 b. In addition, FIG. 26Dillustrates still another example of an exposure pattern 800 d of a lineimage where two dots existing at the boundary with respect to thenon-exposure portion 841 are set to be the high power exposure pixelgroups 843 a and 843 b.

As illustrated FIG. 26B, in the exposure pattern 800 b, among the pixelsof the image portion, the two dots of the both ends in the Y and Z axisdirections are set to the non-exposure portion 841. In addition, thehigh power exposure pixel groups 843 a and 843 b of the exposure pattern800 b are added with the light power values of the pixels which areadjacent to the exposure portion 801 and the non-exposure portion 841and are for the two dots in the Y and Z axis directions.

In this case, since the light power values of the high power exposurepixel group 843 b of the four corners of the exposure portion 801 areadded with the light power values of both the non-exposure portions 841in the Y and Z axis directions, the light power values of the high powerexposure pixel group are four times the light power values of theexposure portion 801 (twice the light power value of the high powerexposure pixel group 843 a).

In addition, in the bent portion of the exposure pattern 800 b, thelight power value of the non-exposure portion 841 is dispersed to pluralplaces. Therefore, the light power value of the high power exposurepixel group 843 c is 1.5 times the light power value of the exposureportion 801.

FIG. 26C illustrates an exposure pattern 800 c in the case where themaximum light power value is set to three times the light power value ofthe standard exposure. As illustrated in the figure, similarly to theexposure pattern 500 c described above, in the exposure pattern 800 c,the high power exposure pixel group 843 d is placed in the exposureportion 801 to disperse the light power values.

FIG. 26D illustrates an exposure pattern 800 d in the case where themaximum light power value is set to twice the light power value of thestandard exposure. As illustrated in the figure, similarly to theexposure pattern 500 d described above, in the exposure pattern 800 d,the light power values are dispersed to the exposure portion 801 a inthe non-exposure portion 841.

Flowchart of Exposure Method

FIG. 27 is a flowchart of the exposure method according to theembodiment. As illustrated in the figure, the light source modulationdata generating circuit 407 performing the exposure method according tothe embodiment receives the image pattern from the informationprocessing device generating the image pattern and detects an arbitrarynumber of pixels (boundary pixels) existing at the boundary portionbetween the image portion and the non-image portion from the imagepattern (Step S101).

The light source modulation data generating circuit 407 performs an edgefolding process to specify the non-exposure portion on the detectedboundary pixels (Step S102).

In order to expose the pixels (adjacent pixels) existing at the boundaryportion between the non-exposure portion and the exposure portion, asthe high power exposure pixel group, the light source modulation datagenerating circuit 407 performs a process of adding the light powervalue for exposing the non-exposure portion (Step S103).

The light source modulation data generating circuit 407 determineswhether or not the light power value of the high power exposure pixelgroup of the exposure pattern is equal to or lower than the maximumlight power value (Step S104).

When the light power value of the high power exposure pixel group isequal to or lower than the maximum light power value (Yes in Step S104),the light source modulation data generating circuit 407 proceeds to theprocess of Step S108.

When the light power value of the high power exposure pixel group ishigher than the maximum light power value (No in Step S104), the lightsource modulation data generating circuit 407 disperse the light powervalue to the high power exposure pixel group of a light power valuehigher than the maximum light power value to generate a new high powerexposure pixel group (Step S105).

The light source modulation data generating circuit 407 determineswhether or not the light power value of the high power exposure pixelgroup of the exposure pattern after the dispersion of the light powervalue is equal to or lower than the maximum light power value (StepS106).

When the light power value of the high power exposure pixel group isequal to or lower than the maximum light power value (Yes in Step S106),the light source modulation data generating circuit 407 proceeds to theprocess of Step S108.

When the light power value of the high power exposure pixel group ishigher than the maximum light power value (No in Step S106), the lightsource modulation data generating circuit 407 allocates the light powervalue to the pixels in the non-exposure portion (outer boundary) whichis adjacent to the high power exposure pixel group of the light powervalue equal to or higher than the maximum light power value andgenerates a new exposure portion (Step S107).

After the setting process of the high power exposure pixel group to theexposure pattern and the dispersion and allocation process of the lightpower value, the light source modulation data generating circuit 407outputs (instructs) the generated exposure pattern to the opticalwriting output unit (Step S108) and terminates the process.

As described above, according to the exposure method according to theembodiment, the images of various shapes can be formed as printmaterials having a high image quality.

Exposure Method (2)

Next, a second exposure method of the image forming method according tothe present invention will be described mainly with respect to adifference from the above-described exposure method.

The second exposure method is another example of a process of addinglight power values of pixels existing at the boundary between the imageportion and the non-image portion to the exposure portion. That is, inthe exposure method according to the second embodiment, the light powervalue addition process is not performed on the pixels where the highpower exposure pixel group reaches the maximum light power value, andthe original exposure pattern is exposed.

FIGS. 28A to 28D are schematic diagrams illustrating examples ofexposure patterns of dot images in the second exposure method.

FIG. 28A illustrates an exposure pattern 900 a of a dot image accordingto the standard exposure. In FIG. 28A, in the exposure pattern 900 a,the entire exposure portion 901 is an image portion which is exposedwith a uniform light power value, and the outside of the frame of theexposure portion 901 is a non-image portion.

Here, as illustrated in FIG. 28A, the exposure pattern 900 a is a squareof 24 dots of 4800 dpi (three dots when converted to 600 dpi).

FIG. 28B illustrates an exposure pattern 900 b of a dot image where twodots existing at the boundary with respect to the non-exposure portion941 are set to a high power exposure pixel group 943 a. In addition,FIG. 28C illustrates another example of an exposure pattern 900 c of aline image where two dots existing at the boundary with respect to thenon-exposure portions 941 a, 941 b are set to high power exposure pixelgroups 943 a, 943 b, and 943 c. In addition, FIG. 28D illustrates stillanother example of an exposure pattern 900 d of a line image where twodots existing at the boundary with respect to the non-exposure portions941 a, 941 b are set to high power exposure pixel groups 943 a, 943 b,and 943 c.

Here, in the exposure method according to the embodiment, the timeconcentration exposure is performed with 200% of the light power valueof the standard exposure, and the upper limit of the maximum light powervalue is set to twice the light power value of the standard exposure.During the exposure, the scanning direction is sequentially repeated inorder of the Y axis direction (left->right) and then the Z axisdirection (up->down).

As illustrated in FIG. 28B, in the exposure pattern 900 b, among thepixels of the image portion, the two dots of the both ends in the Y axisdirection are set to a non-exposure portion 941. In addition, the lightpower value of the high power exposure pixel group 943 a of the exposurepattern 900 b is added with the light power values of the pixels whichare adjacent to the exposure portion 901 and the non-image portion andare for the two dots in the Y axis direction. In this case, since thelight power value of the high power exposure pixel groups 943 a of theboth ends already reach the above-described maximum light power value,the addition of the light power value in the Z axis direction is notperformed.

FIG. 28C illustrates an exposure pattern 900 c in the case where thehigh power exposure pixel group 943 b is placed in the one end in the Zaxis direction of the exposure portion 901. In addition, FIG. 28Dillustrates an exposure pattern 900 d in the case where the high powerexposure pixel groups 943 b are placed in the both ends in the Z axisdirection of the exposure portion 901.

Here, in the case where the high power exposure pixel group 943 b isalso placed in the Z axis direction, since the high power exposure pixelgroup 943 is placed in the Y axis direction, the light power valueexceeds the maximum light power value if the light power value of thenon-exposure portion 941 b is simply added to the light power values ofthe high power exposure pixel groups 943 c at corner portions.

Therefore, in the exposure method according to the embodiment, withrespect to the high power exposure pixel group 943 c of a light powervalue that exceeds the maximum light power value, the light power valueaddition process is not performed.

By doing so, since the maximum light power cannot be exceeded, theexposure method according to the embodiment can be performed on variousimage patterns without exception.

In addition, although the shapes of the exposure patterns 900 c and 900d of FIGS. 28C and 28D appear to be different from the shape of theexposure pattern 900 a having the same shape as the original imagepattern, since the added amount of the light power value is sufficientlysmall in comparison with the beam size, an image such as a protrusionmay not be formed.

That is, according to the exposure method according to the embodiment,approximately, an output image equivalent to the image obtained byexposing corner portions of the dot image with four times the lightpower value can be formed.

In thinning or thickening of an exposure pattern performed in therelated art, since image data are reduced for the thinning orthickening, the image becomes thin.

In addition, in the thinning or thickening of the exposure patternperformed in the related art, if the image data are too much reduced,the data may disappear. Therefore, in order to cope with a small-sizedimage pattern, an exception process is needed, and thus, the coping isdifficult.

On the contrary, in the exposure method according to the embodiment,without reducing the light power value of the exposure pattern accordingto the image data, the light power value is added (moved) to otherpixels as a high power exposure pixel group. That is, according to theexposure method according to the embodiment, without blurring an image,a high-quality image can be formed.

Exposure Method (3)

Next, an example of a third exposure method of the image forming methodaccording to the present invention will be described mainly with respectto a difference from the example of the above-described exposure method.

In the exposure method according to the embodiment, the number of pixelsin the non-exposure portion or the high power exposure pixel group maybe used selectively according to the performance of the image formingapparatus, the image area in the image pattern, the shape (blackcharacter, outline character, line type, image shape, or the like) ofthe image pattern.

FIGS. 29A to 29D are schematic diagrams illustrating examples of lightpower value addition processes for exposure patterns in the thirdexposure method. As illustrated in the figure, in the exposure methodaccording to the embodiment, one to four dots of the exposure patternsof the images formed with 4800 dpi are set to the non-exposure portions,and the light power values are added to other pixels. In the figure, “0”represents the non-exposure pixel (light power value is zero), “1” and“2” represent exposure pixels (coefficient of the light power value isone or two), and “x” represents an arbitrary pixel.

Here, a process of adding only one dot is defined as a 1-dot processmode, and a process of adding two dots is defined as a 2-dot processmode. Hereinafter, according to the number of dots where the light powervalue is added to other dots, the mode name of the process mode isvaried.

FIG. 29A illustrates an example of the addition of a 1-dot foldingprocess. In addition, FIG. 29B illustrates an example of the addition ofa 2-dot folding process. In addition, FIG. 29C illustrates an example ofthe addition of a 3-dot folding process. In addition, FIG. 29Dillustrates an example of the addition of a 4-dot folding process.

As illustrated in FIGS. 29A to 29D, in the exposure method according tothe embodiment, with respect to an arbitrary number of the exposurepixels which are arrayed symmetrically, pattern matching is performed todetermine whether or not the exposure pixels exist at the correspondingpositions when the folding is performed about a virtual symmetric axis.In this manner, the pattern matching is performed, and the light powervalue is added to the pixel of the counter side about the symmetricaxis, so that the numeric value of the exposure pixel of the counterside becomes “2”.

FIGS. 30A and 30B are schematic diagrams illustrating other examples oflight power value addition processes for the exposure patterns in thethird exposure method.

FIG. 30A illustrates an example of the addition of a 3-dot foldingprocess. In addition, FIG. 30B illustrates another example of theaddition of the 3-dot folding process.

As illustrated in FIGS. 30A and 30B, in the exposure method according tothe embodiment, unlike the above-described addition process of exposurepixels which are symmetrically arrayed, even in the case where theexposure pixels do not exist at the corresponding positions when thefolding is performed about a virtual symmetric axis, the additionprocess can be performed.

That is, in the exposure method according to the embodiment, when theaddition process is to be performed, in the case where the exposurepixels of the adding side are already the pixels after the addition ofthe light power value, the addition process may be performed only on theexposure pixels on which the addition can be performed.

More specifically, as illustrated in FIGS. 30A and 30B, in the casewhere 3-dot folding cannot be performed in the 3-dot process mode, aprocess of adding only two dots or a process of adding only one dot canbe performed.

By performing such a process, according to the exposure method accordingto the embodiment, even in the case where the number of exposure pixelson which the addition process according to the folding is to beperformed is set to large, without changing the exposure patterndepending on the non-exposure portion, the images of various shapes canbe formed as print materials having a high image quality.

FIG. 31 is a block diagram illustrating a third exposure method. Asillustrated in the figure, in the exposure method according to theembodiment, the light source modulation data generating circuit 407which receives the image data (original image) generated by aninformation processing device or the like selects which of any one of4-dot to 1-dot folding process modes is to be performed, by a selector.

In the case of the 4-dot folding process mode, the light sourcemodulation data generating circuit 407 performs a conversion process ofperforming searching with respect to whether or not 4 dots can be foldedthrough pattern matching and performing the 4-dot folding when matched.

After the searching through the pattern matching by the 4-dot foldingprocess mode is ended, the light source modulation data generatingcircuit 407 performs a conversion process of performing searching withrespect to whether or not 3-dot folding can be performed through patternmatching and performing the 3-dot folding when matched. The light sourcemodulation data generating circuit 407 sequentially performs suchprocesses up to the 1-dot folding process mode.

Here, in the case where the 2-dot folding process mode is applied, thelight source modulation data generating circuit 407 performs searchingwith respect to whether or not the 2-dot folding can be performed by theselector without performing the processes of the 4-dot and 3-dot foldingprocess modes. In the case where the 2-dot folding can be performed, thelight source modulation data generating circuit 407 performs aconversion process of performing the 2-dot folding.

By doing so, in the exposure method according to the embodiment, the1-dot to 4-dot folding process modes can be performed by onedetermination process.

As described above, according to the exposure method according to theembodiment, the pattern matching is performed in the above-describedconditions and the light power value addition process is performed, sothat the pixels which are added with the light power values can beappropriately processed so as not to be added again.

In addition, in the image forming method according to the presentinvention, at least two types of image qualities of a first imagequality (normal image quality mode) and a second image quality may beallowed to be selected. Here, the first image quality is an imagequality obtained by exposing the entire pixels of the image portion witha target exposure output value for a target exposure time. In addition,the second image quality is an image quality obtained by using theexposure pattern according to the above-described embodiment to exposeat least a group of pixels existing at the boundary with respect to thenon-exposure portion among the pixels constituting the pixels of theimage portion with the light power value higher than the first lightpower value.

Example of Formation of Character Image

Next, an example of application of the exposure method according to theembodiment to a micro-sized (three-point) character image will bedescribed.

FIGS. 32A and 32B are schematic diagrams illustrating exposure patternsof character images according to the exposure method of the embodiment.FIG. 32A illustrates an exposure pattern obtained by performing theexposure in the state where two dots existing at the boundary betweenthe image portion and the non-image portion are set to be thenon-exposure portion and two dots existing at the boundary between theexposure portion and the non-exposure portion are set to be the highpower exposure pixel group. In addition, FIG. 32B illustrates anexposure pattern of a Chinese character “

” which is exposed according to the standard exposure.

FIGS. 33A and 33B are schematic diagrams illustrating exposure patternsof outline character images according to the exposure method of theembodiment. FIG. 33A illustrates an outline exposure pattern of aChinese character “

” which is exposed according to the standard exposure. In addition, FIG.33B illustrates an exposure pattern obtained by performing the exposurein the state where four dots existing at the boundary between the imageportion and the non-image portion are set to be the non-exposure portionand four dots existing at the boundary between the exposure portion andthe non-exposure portion are set to be the high power exposure pixelgroup.

As illustrated in FIGS. 32A, 32B, 33A and 33B, the exposure methodaccording to the embodiment can be applied to color reversed characters(outline characters) as well as normal colored characters.

That is, according to the exposure method according to the embodiment,when the image data are converted into the light source modulation data,even in the case where object information cannot be obtained from theinformation processing device, the exposure patterns of various imagessuch as a character image, an reversed character image, a dither, and aline image can be generated.

In addition, in the exposure method according to the embodiment, theeffect can be enhanced by selecting the folding process modes for theexposure pattern according to the characteristics of the image pattern.

In general, since the periphery of the outlined characters illustratedin FIGS. 33A and 33B is influenced by the exposure, the electric fieldintensity of the white background is reduced, so that the whitebackground can be easily buried in the colored portion. For this reason,in the exposure method according to the embodiment, it is preferablethat the number of pixels of the high power exposure pixel group and thenon-exposure portion be set to be large to increase the light powervalue according to the time concentration exposure.

In addition, in the exposure method according to the embodiment, in thedither portion such as halftone, in the case where textures or artifactsoccur due to influence with other processes, the number of pixels in thehigh power exposure pixel group and the non-exposure portion may bereduced. When the number of pixels which are to be added is one dot,there is almost no disadvantage according to the exposure methodaccording to the embodiment, and the effect of reduction in weakelectric field can be obtained.

For this reason, in the exposure method according to the embodiment, inthe case where a black character, a white character, or dither can beidentified by using tag information identifying a type (character orline) of an object on which the addition process for the high powerexposure pixel group is to be performed, the number of pixels in thehigh power exposure pixel group and the non-exposure portion can beappropriately arranged.

As a specific example, in the case of a normal character or line image,pixels existing at the boundary between the image portion and thenon-image portion are attached with a tag in advance. On the other hand,in the case of a reversed character or reversed line image, pixelsexisting at the boundary between the image portion and the non-imageportion are attached with a tag, and with respect to dither or othersare treated in the same manner as the case where dither is not applied.

Therefore, in each image attached with the tag, a black character or ablack line is set to the 3-dot folding process mode, an outlinecharacter or an outline line is set to the 4-dot folding process mode,and a dither is set to the 2-dot folding process mode in advance, forexample.

First, the light source modulation data generating circuit 407 describedin FIG. 10 detects the boundary pixel between the image portion and thenon-image portion of the exposure pattern and determines from a tag bitof the boundary pixel (information specifying an attribute of an imagepattern) of the boundary pixel whether the tag is zero or one.

Here, in the case where the tag bit is one, the light source modulationdata generating circuit 407 determines that the image is a blackcharacter or a black line and performs the 3-dot folding process mode.

Next, in the case where the tag bit is zero, light source modulationdata generating circuit 407 determines that the image is a whitecharacter or a white line and performs the 4-dot folding process mode.

In the case where the tag bit is neither zero nor one, the light sourcemodulation data generating circuit 407 determines that the image is adither portion and performs the 2-dot folding process mode.

In this manner, in the exposure method according to the embodiment,based on the information such as an image pattern of a received image ora tag bit of the image supplied from the controller, it is recognizedwhether the image is a normal character, a reversed character, or adither portion and the optimal number of folded pixels according to eachimage is set.

That is, according to the exposure method according to the embodiment,since the light power value of the time concentration exposure can bemade stronger or weaker, it is possible to provide an optimal imagecapable of showing the best performance of the image forming apparatus.

First Example

An example of the exposing method of the image forming method accordingto the present invention will be described. In the descriptionhereinafter, for the simplification, image data and an array of pixelsconstituting the image data are indicated by one-dimensional image data.

As illustrated in FIG. 34, in image data 50, a numeric value “1” denotedin pixels constituting an image portion 51 indicates that the pixels areexposed with a predetermined light power value. In addition, in theimage data 50, a numeric value “0” denoted in pixels constituting anon-image portion 52 indicates that the pixels are not exposed(non-exposure pixel). In the exposing method according to the example,in the image data 50, a predetermined pixel among the pixelsconstituting the image portion 51 is specified as a target pixel 53.

After the target pixel 53 is specified, in the exposing method accordingto the example, a boundary pixel group 54 existing in a boundary (edge)between the image portion 51 and the non-image portion 52 is detected.In order to detect the boundary pixel group 54, it is preferable to finda site where the pixel adjacent to the pixel constituting the imageportion 51 and being exposed with a predetermined light power value isthe non-image portion 52.

In the exposing method according to the example, with respect to thedetected boundary pixel group 54, image identification information whichis information identifying a type (character-line image, outlined image,dither-processed image, or the like) of a pattern of the image data 50is acquired. The image identification information is acquired from bothof the pixels constituting the image portion 51 and the pixelsconstituting the non-image portion 52. In the case where the imageidentification information included in two pixels constituting theboundary pixel group 54 is 1 bit for each pixel, the imageidentification information included in each pixel is “0” or “1”. Namely,as illustrated in FIG. 34, as combinations of the image identificationinformation obtained from the boundary pixel group 54 including the twopixels, there are four methods of “Mode1”, “Mode2”, “Mode3”, and“Mode4”.

In the exposing method according to the example, the type of the imagedata 50 is identified based on the image identification information, anda process mode of the exposing method is determined based on theidentified type of the image data 50.

In the exposing method according to the example, the process mode of theexposing process for the image portion 51 including converting of thetarget pixel 53 into a non-exposure pixel or a high power exposure pixelbased on the image identification information is determined. In FIG. 34,the target pixel 53 included in the image portion 51 is exposed with alight power value higher than a light power value (predetermined lightpower value) for other pixels constituting the image portion 51. In FIG.34, the target pixel 53 is exposed with a light power value “2” higherthan a predetermined light power value “1”.

According to the processes described above, in the exposing methodaccording to the example, a degree of a change in latent image electricfield value (slope of the latent image electric field value) is changedin and adjacent to the boundary pixel group 54.

As illustrated in FIG. 35A, the boundary pixel group 54 is convertedinto a high power exposure pixel group 55 where the pixels constitutingthe image portion 51 are exposed with a light power value higher than apredetermined light power value, and a non-exposure pixel group 56 whichconstitutes the image portion 51 and is not exposed. As illustrated inFIG. 35B, in the exposing method according to the example, there is aplurality of modes according to a difference in the numbers of pixels ofthe high power exposure pixel group 55 and the non-exposure pixel group56 included in the boundary pixel group 54. In FIG. 35B, an exposingmethod where the light power values for all the pixels constituting theimage portion 51 included in the image data 50 are constant is referredto as normal exposure.

In the description hereinafter, a process of detecting the boundarypixel group 54 corresponding to the edge portion of the image data 50,partitioning the boundary pixel group 54 into the non-exposure pixelgroup 56 and the high power exposure pixel group 55 and performingexposure is referred to as a folding process. In FIG. 35B, in the casewhere the process mode is Mode1, since both of the high power exposurepixel group 55 and the non-exposure pixel group 56 are 1 dot, theprocess is called a 1-dot folding process. Similarly, in the case wherethe process mode is Mode2, the process is called a 2-dot foldingprocess; and in the case where the process mode is Mode3, the process iscalled a 3-dot folding process.

In the exposing method according to the example, if the light powervalue of the high power exposure pixel group 55 is 200% of apredetermined light power value, an integrated light amount of astandard exposure and an integrated light amount of a TC exposure becomeequivalent to each other.

Here, a TC ratio is defined as follows.

TC ratio=Light power value of high power exposure/Light power value ofstandard exposure

If the TC ratio is less than 200%, the density of the entire imagebecomes slightly low; and if the TC ratio exceeds 200%, the density ofthe entire image becomes slightly high. The TC ratio may be setappropriately according to a required image quality.

In addition, the number of pixels of the high power exposure pixel group55 and the number of pixels of the non-exposure pixel group 56 may beset to be different from each other. Namely, for example, the number ofpixels of the non-exposure pixel group may be set to 1 while the numberof pixels of the high power exposure pixel group 55 may be set to 2 56;the number of pixels of the non-exposure pixel group 56 may be set to 2while the number of pixels of the high power exposure pixel group 55 maybe set to 4; and the number of pixels of the non-exposure pixel group 56may be set to 3 while the number of pixels of the high power exposurepixel group 55 may be set 6. In this case, due to the relationship wherethe number of pixels of the high power exposure pixel group 55 is twotimes the number of pixels of the non-exposure pixel group 56, theintegrated light amount at the time when the TC ratio is 150% and theintegrated light amount of the standard exposure become equivalent toeach other.

FIG. 36 is a graph illustrating latent image electric field intensitydistributions of the exposure patterns (standard exposure (Normal),Mode1, Mode2, and Mode3 illustrated in FIG. 35B) in the first example.The condition of FIG. 36 is obtained in the case where the exposure isperformed on a 2-dot line with 600 dpi (16-dot line with 4800 dpi) witha TC ratio of 200%.

FIG. 37 is an enlarged diagram illustrating a vicinity of a portion ofthe graph of FIG. 36 where the electric field intensity is zero.According to FIGS. 36 and 37, it is found out that there is a differencein latent image electric field intensity according to the process mode.Here, if a degree of steepness of the edge of the image portion 51 iscalculated as a slope of the graph, average slopes per 1 μm are asfollows.

Normal: 4.29E+04 [V/m]

Mode1: 4.36E+04 [V/m]

Mode2: 4.54E+04 [V/m]

Mode3: 4.83E+04 [V/m]

Namely, as the folding amount is increased in the order of Mode1, Mode2,and Mode3, the slope of the latent image electric field intensitybecomes steep. In this manner, in the exposing method according to theexample, since a plurality of the process modes in which the numbers ofpixels of the high power exposure pixel groups 55 are different is setto enable the degree of steepness of the electric field intensity to bechanged, the exposing process corresponding to the type of the imagedata 50 as an object can be appropriately performed.

FIG. 38 is a flowchart illustrating processes of the exposing methodaccording to the first example. The image forming apparatus performingthe exposing method defines a predetermined pixel among the pixelsincluded in the image data 50 as a target pixel 53 and determineswhether or not the target pixel 53 is a pixel in the image portion 51which is to be exposed (Step S201). In the case where the target pixel53 is not a to-be-exposed pixel (No in Step S201), the procedure isended.

In the case where the target pixel 53 is a to-be-exposed pixel, theimage forming apparatus searches for the boundary pixel group 54corresponding to the edge portion between the image portion 51 and thenon-image portion 52 in the image data 50 (Step S202). The image formingapparatus detects the boundary pixel group 54 from the image data 50 andacquires the image identification information stored in the pixels ofthe detected boundary pixel group 54 (Step S203).

The image forming apparatus determines the process mode of the exposingmethod based on the image identification information of the detectedboundary pixel group 54 (Step S204).

The image forming apparatus identifies the high power exposure pixelgroup 55 and the non-exposure pixel group 56 based on the determinedprocess mode and performs the folding process (edge folding process) onthe boundary pixel group 54 and the vicinity thereof (Step S205).

The image forming apparatus determines the light power value of thetarget pixel 53 based on the determined process mode (Step S206) andends the process.

Second Example

Another example of the exposing method of the image forming methodaccording to the present invention will be described.

In the exposing method according to the example, the acquired imageidentification information is included in the pixel separated away fromthe boundary pixel group 54 by at least a sum of the number ofconsecutive groups each consisting of pixels in one column and includedin the non-exposure pixel group 56 and the number of consecutive groupseach consisting of pixels in one column and included in the high powerexposure pixel group 55. Namely, it is preferable that the imageidentification information be included in the pixels up to the pixelseparated away from the edge of the image portion 51 by two times ormore the maximum folding amount of the boundary pixel group 54 (amountof the high power exposure pixel group 55).

For example, in the case where the number of groups each consisting ofpixels in one column in the non-exposure pixel groups 56 is 2 and thenumber of groups each consisting of pixels in one column in the highpower exposure pixel group 55 is 4, the image identification informationof at least six pixel groups is required.

FIG. 39A illustrates an example of the image portion 51 and thenon-image portion 52 in the image data 50 of the image pattern of acharacter-line image in the exposing method according to the example. Inaddition, FIG. 39B illustrates the image identification information 51Tof the image data, and FIG. 39C illustrates the high power exposurepixel group 55 and the non-exposure pixel group 56.

FIG. 40A illustrates an example of the image portion 51 and thenon-image portion 52 in the image data 50 of the image pattern of anoutlined character-line image of in the exposing method according to theexample. In addition, FIG. 40B illustrates the image identificationinformation 51T of the image data of the outlined character-line image,and FIG. 40C illustrates the high power exposure pixel group 55 and thenon-exposure pixel group 56.

FIG. 41A illustrates an example of the image portion 51 and thenon-image portion 52 in the image data 50 of the image pattern of adither-processed image in the exposing method according to the example.FIG. 41B illustrates the image identification information 51T of theimage data 50 of a dither-processed image, and FIG. 41C illustrates thehigh power exposure pixel group 55 and the non-exposure pixel group 56.

In the examples of FIGS. 39 to 41, in the case where the number ofgroups each consisting of pixels in one column in the non-exposure pixelgroups 56 is 2 and the number of groups each consisting of pixels in onecolumn in the high power exposure pixel group 55 is 4, the imageidentification information 51T of at least six pixel groups is required.In addition, in the case where the number of groups each consisting ofpixels in one column in the non-exposure pixel groups 56 is 3 and thenumber of groups each consisting of pixels in one column in the highpower exposure pixel group 55 is 3, the image identification information51T of at least six pixel groups is required.

In the case where the number of groups each consisting of pixels in onecolumn in the non-exposure pixel groups 56 is 4 and the number of groupseach consisting of pixels in one column in the high power exposure pixelgroup 55 is 4, the image identification information 51T of at leasteight pixel groups is required. In addition, in the case where thenumber of groups each consisting of pixels in one column in thenon-exposure pixel groups 56 is 4 and the number of groups eachconsisting of pixels in one column in the high power exposure pixelgroup 55 is 8, the image identification information 51T of at leasttwelve pixel groups is required.

Third Example

Still another example of the exposing method of the image forming methodaccording to the present invention will be described with reference toFIGS. 42A to 42F. In the example, the case where a sum of the number ofpixels of the high power exposure pixel group 55 and the number ofpixels of the non-exposure pixel group 56 is 6 is exemplified todescribe a method of detecting a boundary pixel group of image data inthe exposing method of the image forming method according to the presentinvention.

In the exposing method according to the example, the target pixel 53 atthe center of the image data 50 is set as a reference, and cross-shapedpattern matching illustrated in FIG. 42A to FIG. 42F is sequentiallyperformed.

In the pattern matching of FIG. 42A, four pixels adjacent to each otherin the horizontal and vertical directions from the target pixel 53 areset as reference pixels 57, and it is determined whether or not there isan edge portion. If an edge is detected from the detected referencepixels 57, the distance from the target pixel 53 to the edge isidentified as one pixel. At this time, if no edge is detected from anyone of the four reference pixels 57, the pattern matching of FIG. 42B isperformed.

In the pattern matching of FIG. 42B, in comparison with FIG. 42A,reference pixels 57 are set to be separated away from the target pixel53 by one pixel in the horizontal and vertical directions, and it isdetermined whether or not there is an edge portion. If an edge isdetected from the detected reference pixels 57, the distance from thetarget pixel 53 to the edge is identified as two pixels. At this time,if no edge is detected from any one of the detected reference pixels 57,the pattern matching of FIG. 42C is performed.

In the pattern matching of FIG. 42C, in comparison with FIG. 42B,reference pixels 57 are set to be separated away from the target pixel53 by two pixels in the horizontal and vertical directions, and it isdetermined whether or not there is an edge portion. If an edge isdetected from the detected reference pixels 57, the distance from thetarget pixel 53 to the edge is identified as three pixels. At this time,if no edge is detected from any one of the detected reference pixels 57,the pattern matching of FIG. 42D is performed.

Until an edge is detected, such pattern matching is performed apredetermined number of times N from the target pixel 53 to thereference pixels 57 separated by five pixels away from the target pixel,for example, as illustrated in FIG. 42F.

The number of times N of performing the pattern matching N is a sum ofthe number of dots of the high power exposure pixel group 55 and thenumber of dots of the non-exposure pixel group 56. Namely, in the casewhere the number of the high power exposure pixel group 55 is 3 dots andthe number of the non-exposure pixel group 56 is 3 dots, N=6.

In addition, in the case where the patterning matching is performed Ntimes but no edge portion is detected from any one of the referencepixels 57, it is determined that there is no edge of the image portion51 adjacent to the target pixel 53, and a process corresponding to thissituation is performed.

As described above, in the exposing method according to the example, thedistance from the target pixel of the image data to the edge can bedetected sequentially in the order from the closet four sites in thevertical and horizontal directions.

Fourth Example

Further still another example of the exposing method of the imageforming method according to the present invention will be described withreference to a flowchart illustrated in FIG. 43. In exposing methodaccording to the example, with respect to arbitrary two-dimensionalimage data, three types of image patterns are identified based on 1-bitimage identification information.

In the exposing method according to the example, an image character-lineimage portion (black character), an outlined character-line image (whitecharacter), a dither-processed image (dither), and a not-subject imageare identified.

When the light power value of the image data is 1, if the imageidentification information of the target pixel is 1, the image patternis a black character. When the light power value of the target pixel is1, the portion is an image portion; and when the light power value ofthe target pixel is 0, the portion is a non-image portion. When theimage identification information is 1, there is a tag; and when theimage identification information is 0, there is no tag. In addition, Xis assumed to be an arbitrary value.

The image forming apparatus determines whether or not the light powervalue of the target pixel is 0 (Step S301). If the light power value ofthe target pixel is 0 (Yes in Step S301), the target pixel is anon-image portion irrespective of the value of the image identificationinformation, so that 0 is output (Step S305), and the procedure isended.

If the light power value of the target pixel is 1 (No in Step S301), itis determined whether or not the value of the image identificationinformation is 1 (Step S302). If the value of the image identificationinformation is 1 (Yes in Step S302), it can be determined that thetarget pixel is a black character (Step S306).

In the case where the light power value of the target pixel is 1 and thevalue of the image identification information is not 1 (No in StepS302), the image forming apparatus specifies four pixels adjacent to thetarget pixel as reference pixels. The image forming apparatus determineswhether or not a pixel corresponding to (light power value, imageidentification information)=(0, 0), that is, information specifying adither image is included in any one of the reference pixels (Step S303).If a pixel corresponding to (light power value, image identificationinformation)=(0, 0) is included in any one of the reference pixels (Yesin Step S303), it can be determined that the image is a dither image(Step S307).

In the case where all the reference pixels are not pixels correspondingto (light power value, image identification information)=(0, 0) (No inStep S303), it is determined whether or not any one of the referencepixels is a pixel corresponding to (X, 1), that is, informationspecifying a white character (Step S304). If a pixel corresponding to(light power value, image identification information)=(X, 1) is includedin any one of the reference pixels (Yes in Step S304), it can bedetermined that the image is a white character (Step S308).

In the case where all the reference pixels are not pixels correspondingto (light power value, image identification information)=(X, 1) (No inStep S304), four pixels separated away from the target pixel by onepixel in the horizontal and vertical directions are set as referencepixels, and the processes of S303 and S304 are repeated an arbitrarynumber of times N (Step S309).

When the processes of S303 and S304 are repeated an arbitrary number oftimes N (Yes in Step S309), the procedure is ended.

By performing the procedure as described above, in the exposing methodaccording to the example, the three types of the images can beidentified based on the 1-bit image identification information.

Configuration of Electrostatic Latent Image Measurement Device

Next, a configuration of the electrostatic latent image measurementdevice capable of checking an electrostatic latent image state formed bythe exposure method according to the embodiment will be described.

FIG. 44 is a central circuit diagram illustrating the electrostaticlatent image measurement device.

The electrostatic latent image measurement device 300 includes a chargedparticle irradiation system 400, an optical scanning device 1010, asample stage 401, a detector 402, an LED 403, a control system (notillustrated), an ejection system (not illustrated), and a driving powersource (not illustrated).

The charged particle irradiation system 400 is disposed inside a vacuumchamber 340. Here, the charged particle irradiation system 400 includesan electron gun 311, an extraction electrode 312, an accelerationelectrode 313, a condenser lens 314, a beam blanker 315, and a partitionplate 316. In addition, the charged particle irradiation system 400includes a movable aperture stop 317, a stigmator 318, a scanning lens319, and an objective lens 320.

In addition, in the description hereinafter, the optical axis directionof each lens is described as a c-axis direction, and two directionsperpendicular to each other in the plane perpendicular to the c-axisdirection are described as an a-axis direction and a b-axis direction.

The electron gun 311 generates an electron beam as a charged particlebeam.

The extraction electrode 312 is disposed in the −c direction from theelectron gun 311 to control the electron beam generated by the electrongun 311.

The acceleration electrode 313 is disposed in the −c direction from theextraction electrode 312 to control energy of the electron beam.

The condenser lens 314 is disposed in the −c direction from theacceleration electrode 313 to converge the electron beam.

The beam blanker 315 is disposed in the −c direction from the condenserlens 314 to turn on/off the electron beam irradiation.

The partition plate 316 is disposed in the −c direction from the beamblanker 315 and has an opening at the center thereof.

The movable aperture stop 317 is disposed in the −c direction from thepartition plate 316 to adjust a beam diameter of the electron beam thathas passed through the opening of the partition plate 316.

The stigmator 318 is disposed in the −c direction from the movableaperture stop 317 to correct astigmatism.

The scanning lens 319 is disposed in the −c direction from the stigmator318 to deflect the electron beam that has passed through the stigmator318, in an ab plane.

The objective lens 320 is disposed in the −c direction from the scanninglens 319 to converge the electron beam that has passed through thescanning lens 319. The electron beam that has passed through theobjective lens 320 passes through a beam emitting opening portion 321and irradiates a surface of the sample 323.

Each lens or the like is connected to the driving power source (notillustrated).

In addition, the charged particles denote particles influenced by anelectric field or a magnetic field. Here, as the beam of irradiating thecharged particles, for example, ion beams may be used instead of theelectron beam. In this case, a liquid metal ion gun or the like is usedinstead of the electron gun.

The sample 323 is a photoconductor and includes a conductive supportingbody, a charge generation layer (CGL) and a charge transport layer(CTL).

The charge generation layer includes a charge generation material (CGM)and is formed in a surface of the +c side of the conductive supportingbody. The charge transport layer is formed in the surface of the +c sideof the charge generation layer.

When the sample 323 is exposed in the state where the surface (surfacein the +c side) is charged, light is absorbed by the charge generationmaterial of the charge generation layer, so that charge carriers havingtwo polarities of positive and negative polarities are generated. Due tothe electric field, some of the carriers are injected to the chargetransport layer, and others thereof are injected to the conductivesupporting body.

Due to the electric field, the carriers injected to the charge transportlayer are moved to the surface of the charge transport layer and arecoupled with the charges of the surface to disappear. Accordingly, onthe surface (surface in the +c side) of the sample 323, a chargedistribution, that is, an electrostatic latent image is formed.

The optical scanning device 1010 includes a light source, a couplinglens, an opening plate, a cylindrical lens, a polygon mirror, and ascanning optical system. In addition, the optical scanning device 1010also includes a scanning mechanism (not illustrated) for scanning thelight with respect to the direction parallel to the rotation axis of thepolygon mirror.

The surface of the sample 323 is irradiated with the light emitted fromthe optical scanning device 1010 through a reflecting mirror 372 and awindow glass 368.

On the surface of the sample 323, the irradiation position of the lightemitted from the optical scanning device 1010 is varied in the twodirections perpendicular to each other on the plane perpendicular to thec-axis direction due to deflection in the polygon mirror and deflectionin the scanning mechanism. At this time, the varying direction of theirradiation position due to the deflection in the polygon mirror is themain-scanning direction, and the varying direction of the irradiationposition due to the deflection in the scanning mechanism is thesub-scanning direction. Here, the a-axis direction is set as themain-scanning direction, and the b-axis direction is set as thesub-scanning direction.

In this manner, the electrostatic latent image measurement device 300can two-dimensionally scan the surface of the sample 323 with the lightemitted from the optical scanning device 1010. That is, theelectrostatic latent image measurement device 300 can form atwo-dimensional electrostatic latent image on the surface of the sample323.

Incidentally, the optical scanning device 1010 is installed outside thevacuum chamber 340 so that vibration or electromagnetic waves generatedby a driving motor of the polygon mirror does not influence a trajectoryof the electron beam. Therefore, the influence of disturbance on themeasurement result can be suppressed.

The detector 402 is disposed adjacent to the sample 323 to detectsecondary electrons of the sample 323.

The LED 403 is disposed adjacent to the sample 323 to emit light forillumination of the sample 323. The LED 403 is used to erase the chargesremaining on the surface of the sample 323 after the measurement.

In addition, an optical housing retaining the scanning optical systemmay be constructed to cover the entire scanning optical system so as toblock external light (harmful light) incident into the vacuum chamber.

In the scanning optical system, the scanning lens has fθcharacteristics, and when an optical polarizer is rotated at a certainspeed, the light beam is designed to be moved at a approximatelyconstant speed with respect to an image plane. In addition, in thescanning optical system, the beam spot diameter is also designed to beapproximately constant during the scanning.

In the electrostatic latent image measurement device 300, since thescanning optical system is disposed to be separated from the vacuumchamber, there is small influence of direct propagation of the vibrationgenerated from the driving of an optical deflector such as a polygontype scanner to the vacuum chamber 340.

In addition, it is possible to obtain higher anti-vibration effect byinstalling anti-vibration measures such as a damper to a structure (notillustrated) for retaining the scanning optical system.

In the electrostatic latent image measurement device 300, the scanningoptical system is installed to enable any arbitrary latent image patternincluding a line pattern to be formed in a generating line direction ofthe photoconductor.

In addition, in order to form a latent image pattern at a predeterminedposition, the synchronization detection sensor 26 for sensing a scanningbeam of an optical deflecting unit may be installed.

In addition, the shape of the sample may be a planar surface or a curvedsurface.

Electrostatic Latent Image Measurement Method

Next, an electrostatic latent image measurement method will bedescribed.

FIG. 45 is a schematic diagram illustrating a relationship between theacceleration voltage and the charging. First, during the electrostaticlatent image measurement, in the electrostatic latent image measurementdevice 300, the sample 323 of the photoconductor is irradiated with theelectron beam.

As illustrated in FIG. 45, as the acceleration voltage |Vacc| which isthe voltage applied to the acceleration electrode 313, a voltage higherthan the voltage in which a secondary electron emission ratio of thesample 323 becomes one is set. By setting the acceleration voltage inthis manner, since the amount of the incident electrons is larger thanthe amount of the emission electrons in the sample 323, the electronsare accumulated in the sample 323, so that charge-up occurs. As aresult, in the electrostatic latent image measurement device 300, thesurface of the sample 323 can be charged uniformly with negativecharges.

FIG. 46 is a graph illustrating a relationship between the accelerationvoltage and the charge potential. As illustrated in the figure, there isa certain relationship between the acceleration voltage and the chargepotential. For this reason, in the electrostatic latent imagemeasurement device 300, the acceleration voltage and the irradiationtime are appropriately set to enable the same charge potential as thecharge potential of the photoconductor drum 1030 in the image formingapparatus 1000 to be formed on the surface of the sample 323.

Incidentally, as an irradiation current is large, a target chargepotential can be achieved in a short time. Therefore, in this case, theirradiation current is set to be several nano amperes (nA).

Subsequently, in the electrostatic latent image measurement device 300,the amount of electrons which are incident on the sample 323 is set to1/100 times to 1/1000 times so that the electrostatic latent image canbe observed.

The electrostatic latent image measurement device 300 controls theoptical scanning device 500 to two-dimensionally perform opticalscanning on the surface of the sample 323 and forms the electrostaticlatent image on the sample 323. In addition, the optical scanning device500 is controlled such that the light spot having a desired beamdiameter and beam profile is formed on the surface of the sample 323.

By the way, although the exposure energy necessary for forming theelectrostatic latent image is defined according to the sensitivitycharacteristics of the sample, the exposure energy is typically about 2to 10 mJ/m². In addition, in some cases, in the case of a sample of lowsensitivity, the necessary exposure energy is 10 mJ/m² or more. That is,the charge potential or the necessary exposure energy is set inaccordance with the photosensitivity characteristics of the sample orthe process conditions. Here, the exposure conditions of theelectrostatic latent image measurement device 300 are set to be the sameas the exposure conditions in accordance with the image formingapparatus 1000.

Therefore, in such a case, the environment of electrostatic field or thetrajectory of electrons is calculated in advance, and the detectionresult is corrected based on the calculation result, so that it ispossible to obtain a profile of the electrostatic latent image at a highaccuracy.

As described above, by using the electrostatic latent image measurementdevice 300, it is possible to obtain a charge distribution of anelectrostatic latent image, a surface potential distribution, anelectric field intensity distribution, and an electric field intensityin the direction perpendicular to the sample surface at the respectivehigh accuracies.

Effect

As described above, according to the exposure method according to theembodiment, the following effects can be obtained.

According to the exposure method according to the embodiment, byperforming the time concentration exposure (TC exposure) where theexposure is performed with a strong light power for a short time, it ispossible to obtain the effect equivalent to the effect of a spatialconcentration exposure (reduction of the diameter of the beam).According to the exposure method according to the embodiment, since anelectrostatic latent image which has a small area and a certain depthcan be formed, it is possible to form a high-resolution image.

In addition, according to the exposure method according to theembodiment, it is possible to selectively use conditions such asexposure patterns according to an image or conditions of theenvironment.

In addition, in the exposure method according to the embodiment, theexposure pattern is concentrated on the inner side (central portion) ofthe input image data, and the exposure is performed with the light powervalue which is stronger than the light power value of the standardexposure. Therefore, according to the exposure method according to theembodiment, even with respect to a micro-sized image pattern, an imagecomplying with the target image with a desired image density can beoutput.

In addition, according to the exposure method according to theembodiment, by setting the pixels existing at the boundary between theimage portion and the non-image portion to be the high power exposurepixel group and increasing the light power value of the pixels of theexposure portion, the time concentration exposure can be applied tovarious image patterns.

In addition, according to the exposure method according to theembodiment, in the case where the light power value of the high powerexposure pixel group exceeds a predetermined maximum light power value,the light power is dispersed to the adjacent image portion pixels, andthe exposure is performed. Therefore, according to the exposure methodaccording to the embodiment, even in the case where the maximum lightpower value cannot be set to be high, the image density is maintained,so that it is possible to implement a high image quality.

In addition, in the exposure method according to the embodiment, thepixels of the image portion existing at the boundary between the imageportion and the non-image portion are converted into the non-exposureportion and, simultaneously, the light power value comparable to thenon-exposure portion is added to the pixels of the image portionadjacent to the high power exposure pixel group to perform the exposure.Thereby, according to the exposure method according to the embodiment,the time concentration exposure can be applied to various image patternssuch as a character image, a reversed character image, a dither, and aline image.

In addition, according to the exposure method according to theembodiment, since the light power value is not added in the case wherethe light power value of the high power exposure pixel group exceeds themaximum light power value, an exception process does not need to beperformed so that the time concentration exposure can be applied tovarious images with a simple, easy process.

In addition, according to the exposure method according to theembodiment, it is possible to perform the time concentration exposure inoptimal conditions, since the process only for the number of pixels onwhich addition can be performed is performed in the case where, when thelight power value is to be added to the high power exposure pixel group,the high power exposure pixel group is already added with the lightpower value.

In addition, in the exposure method according to the embodiment, thelight source modulation data generating circuit 407 recognizes the imagestate based on information such as a tag bit supplied from the imageprocessing circuit or the controller and sets the optimal number offolding pixels according to each image. Therefore, according to theexposure method according to the embodiment, it is possible to providean optimal image capable of showing the best performance of the imageforming apparatus.

Particularly, according to the exposure method according to theembodiment, at the places such as a dither portion where the imagepatterns are adjacent to each other and there are many areas where thelatent image electric field is easily weakened, the weak electric fieldarea is reduced, so that it is possible to improve dot reproducibility.

In addition, according to the exposure method according to theembodiment, since the image quality in the electrostatic latent imagestage is improved, a high image quality can be stably realized in themicro-sized character image, particularly, the reversed character image.

In addition, according to the exposure method according to theembodiment, since the method is simple and easy, the method can beperformed on various images at a high speed.

In addition, according to the exposure method according to theembodiment, since the integrated light amount of the standard exposureand the integrated light amount of the time concentration exposure canbe equal to each other by intentionally strengthening the maximum lightpower value using PM+PWM modulation, it is possible to increase aresolution by forming a deep latent image.

In addition, according to the image forming apparatus according to theembodiment, the exposure method according to the embodiment isvisualized by developing, so that it is possible to provide an imageforming apparatus having a high density and a high image quality.

The image forming apparatus according to the embodiment is appropriatefor an image forming apparatus including a multi-beam scanning opticalsystem, particularly, VCSEL or the like.

According to an embodiment, it is possible to form a high-quality imageby an image pattern including an image portion composed of a pluralityof pixels and a non-image portion.

The above-described embodiments are illustrative and do not limit thepresent invention. Thus, numerous additional modifications andvariations are possible in light of the above teachings. For example, atleast one element of different illustrative and exemplary embodimentsherein may be combined with each other or substituted for each otherwithin the scope of this disclosure and appended claims. Further,features of components of the embodiments, such as the number, theposition, and the shape are not limited the embodiments and thus may bepreferably set. It is therefore to be understood that within the scopeof the appended claims, the disclosure of the present invention may bepracticed otherwise than as specifically described herein.

The method steps, processes, or operations described herein are not tobe construed as necessarily requiring their performance in theparticular order discussed or illustrated, unless specificallyidentified as an order of performance or clearly identified through thecontext. It is also to be understood that additional or alternativesteps may be employed.

What is claimed is:
 1. An image forming method of exposing a surface ofan image bearer with light according to an image pattern including animage portion and a non-image portion to form an electrostatic latentimage corresponding to the image pattern, the image portion including aplurality of pixels, the method comprising: setting, among the pixelsconstituting the image portion, at least a group of pixels existing at aboundary with respect to the non-image portion as a non-exposure pixelgroup; setting, among the pixels constituting the image portion, atleast a group of pixels existing at a boundary with respect to thenon-exposure pixel group as a high power exposure pixel group whereexposure is performed with light of a higher light power value than apredetermined light power value required for exposing the image portion;specifying, among the pixels constituting the image portion, apredetermined pixel as a target pixel; specifying, among the pixelsconstituting the image portion, a group of pixels existing at a boundarywith respect to the non-image portion close to the target pixel as aboundary pixel group; acquiring image identification informationidentifying a type of the image pattern from the pixels constituting theimage portion; and specifying a light power value of the target pixelbased on the image identification information.
 2. The image formingmethod according to claim 1, wherein the image identificationinformation is acquired from a pixel separated from the boundary pixelgroup by at least a sum of number of consecutive groups each consistingof pixels in one column and included in the non-exposure pixel group andnumber of consecutive groups each consisting of pixels in one column andincluded in the high power exposure pixel group.
 3. The image formingmethod according to claim 1, wherein a distance from the boundary pixelgroup to the target pixel is identified based on a pixel which isclosest to the boundary pixel group among pixels adjacent to the targetpixel.
 4. The image forming method according to claim 1, wherein theimage identification information is information which can specify atleast three or more types of the image patterns.
 5. The image formingmethod according to claim 1, wherein a predetermined light power valuerequired for exposing the group of pixels and a light power valuerequired for exposing the non-exposure pixel group are added to exposethe high power exposure pixel group.
 6. The image forming methodaccording to claim 1, wherein a light power value of the high powerexposure pixel group is set according to number of adjacent non-exposurepixel groups.
 7. The image forming method according to claim 1, wherein,in the high power exposure pixel group, a maximum light power value perpixel is set, and a group of pixels which are adjacent to a group ofpixels which exceed the maximum light power value, among the high powerexposure pixel groups, is exposed with a predetermined light powervalue.
 8. The image forming method according to claim 7, wherein thegroup of pixels which are adjacent to the group of pixels which exceedthe maximum light power value is exposed with the maximum light powervalue.
 9. The image forming method according to claim 1, wherein thehigh power exposure pixel group is exposed for a time which is shorterthan a predetermined time.
 10. The image forming method according toclaim 1, wherein an integrated light amount of light exposing pixelswhich are exposed in the image portion is equal to an integrated lightamount of light power values in a case where entire pixels constitutingthe image portion are exposed for a predetermined time with light of thepredetermined light power value.
 11. An image forming apparatusconfigured to expose a surface of an image bearer with light accordingto an image pattern including an image portion and a non-image portionto form an electrostatic latent image corresponding to the imagepattern, comprising: a light source configured to perform irradiationwith the light; a light source driving unit configured to generate alight source driving current for driving the light source; and anoptical system configured to guide light emitted from the light sourceto the latent image bearer, the image portion including a plurality ofpixels, the light source driving unit being configured to, based oninformation specifying an attribute of the image pattern, specify, amongthe pixels constituting the image portion, at least a group of pixelsexisting at a boundary with respect to the non-image portion as anon-exposure pixel group, specify, among the pixels constituting theimage portion, at least a group of pixels existing at a boundary withrespect to the non-exposure pixel group as a high power exposure pixelgroup where exposure is performed with light of a higher light powervalue than a predetermined light power value required for exposing theimage portion, and drive the light source with power valuescorresponding to the specified high power exposure pixel group and thespecified non-exposure pixel group.
 12. The image forming apparatusaccording to claim 11, wherein the light source driving unit isconfigured to specify, among the pixels constituting the image portion,a predetermined pixel as a target pixel, specify, among the pixelsconstituting the image portion, a group of pixels existing at a boundarywith respect to the non-image portion close to the target pixel as aboundary pixel group, acquire image identification informationidentifying a type of the image pattern from a pixel constituting theimage portion, and specify a light power value of the target pixel basedon the image identification information.
 13. The image forming apparatusaccording to claim 12, wherein the image identification information isacquired from a pixel separated from the boundary pixel group by a sumof number of consecutive groups each consisting of pixels in one columnincluded in at least the non-exposure pixel group and number ofconsecutive groups each consisting of pixels in one column included inthe high power exposure pixel group.
 14. The image forming apparatusaccording to claim 12, wherein a distance from the boundary pixel groupto the target pixel is identified based on a pixel which is closest tothe boundary pixel group among pixels adjacent to the target pixel. 15.The image forming apparatus according to claim 12, wherein the imageidentification information is information which can specify at leastthree or more types of the image patterns.
 16. An image formingapparatus configured to expose a surface of an image bearer with lightaccording to an image pattern including an image portion and a non-imageportion and be capable of selecting one from at least two types of imagequalities of an output, the image portion including a plurality ofpixels, the image forming apparatus being configured to set, among thepixels constituting the image portion, at least a group of pixelsexisting at a boundary with respect to the non-image portion as anon-exposure pixel group, set a light power value for exposing the imageportion to a first light power value in a case where a first imagequality is selected, and expose, among the pixels constituting the imageportion, at least a group of pixels existing at a boundary with respectto the non-exposure pixel group with a light power value which is higherthan the first light power value in a case where a second image qualityis selected.